diff --git a/src/usher_pipeline/gene_mapping/__init__.py b/src/usher_pipeline/gene_mapping/__init__.py
index 3fa5a13..d9960d7 100644
--- a/src/usher_pipeline/gene_mapping/__init__.py
+++ b/src/usher_pipeline/gene_mapping/__init__.py
@@ -13,6 +13,11 @@ from usher_pipeline.gene_mapping.universe import (
     fetch_protein_coding_genes,
     GeneUniverse,
 )
+from usher_pipeline.gene_mapping.validator import (
+    MappingValidator,
+    ValidationResult,
+    validate_gene_universe,
+)
 
 __all__ = [
     "GeneMapper",
@@ -20,4 +25,7 @@ __all__ = [
     "MappingReport",
     "fetch_protein_coding_genes",
     "GeneUniverse",
+    "MappingValidator",
+    "ValidationResult",
+    "validate_gene_universe",
 ]
diff --git a/src/usher_pipeline/gene_mapping/validator.py b/src/usher_pipeline/gene_mapping/validator.py
new file mode 100644
index 0000000..644f3e2
--- /dev/null
+++ b/src/usher_pipeline/gene_mapping/validator.py
@@ -0,0 +1,224 @@
+"""Validation gates for gene mapping quality control.
+
+Provides validation for mapping results and gene universe data quality.
+Enforces configurable success rate thresholds and produces actionable reports.
+"""
+
+import logging
+from dataclasses import dataclass, field
+from datetime import datetime
+from pathlib import Path
+
+from usher_pipeline.gene_mapping.mapper import MappingReport
+
+logger = logging.getLogger(__name__)
+
+
+@dataclass
+class ValidationResult:
+    """Result of a validation check.
+
+    Attributes:
+        passed: Whether validation passed
+        messages: List of validation messages (warnings, errors)
+        hgnc_rate: HGNC mapping success rate (0-1)
+        uniprot_rate: UniProt mapping success rate (0-1)
+    """
+    passed: bool
+    messages: list[str] = field(default_factory=list)
+    hgnc_rate: float = 0.0
+    uniprot_rate: float = 0.0
+
+
+class MappingValidator:
+    """Validator for gene ID mapping results.
+
+    Enforces configurable success rate thresholds and produces validation reports.
+    """
+
+    def __init__(
+        self,
+        min_success_rate: float = 0.90,
+        warn_threshold: float = 0.95
+    ):
+        """Initialize mapping validator.
+
+        Args:
+            min_success_rate: Minimum HGNC mapping success rate to pass (default: 0.90)
+            warn_threshold: Success rate below this triggers warning (default: 0.95)
+        """
+        self.min_success_rate = min_success_rate
+        self.warn_threshold = warn_threshold
+        logger.info(
+            f"Initialized MappingValidator: min_rate={min_success_rate}, "
+            f"warn_threshold={warn_threshold}"
+        )
+
+    def validate(self, report: MappingReport) -> ValidationResult:
+        """Validate gene mapping results.
+
+        Checks if HGNC mapping success rate meets minimum threshold.
+        Issues warning if rate is below warn_threshold but above min_success_rate.
+
+        Args:
+            report: MappingReport from batch mapping operation
+
+        Returns:
+            ValidationResult with pass/fail status and messages
+        """
+        messages: list[str] = []
+        hgnc_rate = report.success_rate_hgnc
+        uniprot_rate = report.success_rate_uniprot
+
+        # Check HGNC success rate
+        if hgnc_rate < self.min_success_rate:
+            messages.append(
+                f"FAILED: HGNC mapping success rate {hgnc_rate:.1%} is below "
+                f"minimum threshold {self.min_success_rate:.1%}"
+            )
+            messages.append(
+                f"Mapped {report.mapped_hgnc}/{report.total_genes} genes to HGNC symbols"
+            )
+            messages.append(
+                f"Unmapped genes: {len(report.unmapped_ids)} "
+                f"(first 10: {report.unmapped_ids[:10]})"
+            )
+            passed = False
+        elif hgnc_rate < self.warn_threshold:
+            messages.append(
+                f"WARNING: HGNC mapping success rate {hgnc_rate:.1%} is below "
+                f"warning threshold {self.warn_threshold:.1%}"
+            )
+            messages.append(
+                f"Mapped {report.mapped_hgnc}/{report.total_genes} genes to HGNC symbols"
+            )
+            messages.append(
+                f"Consider reviewing {len(report.unmapped_ids)} unmapped genes"
+            )
+            passed = True
+        else:
+            messages.append(
+                f"PASSED: HGNC mapping success rate {hgnc_rate:.1%} "
+                f"({report.mapped_hgnc}/{report.total_genes} genes)"
+            )
+            passed = True
+
+        # Report UniProt stats (informational only, not used for pass/fail)
+        messages.append(
+            f"UniProt mapping: {uniprot_rate:.1%} "
+            f"({report.mapped_uniprot}/{report.total_genes} genes)"
+        )
+
+        logger.info(
+            f"Validation result: {'PASSED' if passed else 'FAILED'} "
+            f"(HGNC: {hgnc_rate:.1%}, UniProt: {uniprot_rate:.1%})"
+        )
+
+        return ValidationResult(
+            passed=passed,
+            messages=messages,
+            hgnc_rate=hgnc_rate,
+            uniprot_rate=uniprot_rate,
+        )
+
+    def save_unmapped_report(
+        self,
+        report: MappingReport,
+        output_path: Path
+    ) -> None:
+        """Save list of unmapped genes to file for manual review.
+
+        Args:
+            report: MappingReport containing unmapped gene IDs
+            output_path: Path to output file
+        """
+        output_path = Path(output_path)
+        output_path.parent.mkdir(parents=True, exist_ok=True)
+
+        timestamp = datetime.now().strftime("%Y-%m-%d %H:%M:%S")
+        unmapped_count = len(report.unmapped_ids)
+
+        with output_path.open('w') as f:
+            f.write(f"# Unmapped Gene IDs\n")
+            f.write(f"# Generated: {timestamp}\n")
+            f.write(f"# Total unmapped: {unmapped_count}\n")
+            f.write(f"# Success rate: {report.success_rate_hgnc:.1%}\n")
+            f.write(f"#\n")
+            for gene_id in report.unmapped_ids:
+                f.write(f"{gene_id}\n")
+
+        logger.info(
+            f"Saved {unmapped_count} unmapped gene IDs to {output_path}"
+        )
+
+
+def validate_gene_universe(genes: list[str]) -> ValidationResult:
+    """Validate gene universe data quality.
+
+    Checks:
+    - Gene count is in expected range (19,000-22,000)
+    - All gene IDs start with ENSG (Ensembl format)
+    - No duplicate gene IDs
+
+    Args:
+        genes: List of gene IDs to validate
+
+    Returns:
+        ValidationResult with validation status and messages
+    """
+    messages: list[str] = []
+    passed = True
+
+    gene_count = len(genes)
+    MIN_GENES = 19000
+    MAX_GENES = 22000
+
+    # Check gene count
+    if gene_count < MIN_GENES:
+        messages.append(
+            f"FAILED: Gene count {gene_count} is below minimum {MIN_GENES}. "
+            "This may indicate missing data or incomplete query."
+        )
+        passed = False
+    elif gene_count > MAX_GENES:
+        messages.append(
+            f"FAILED: Gene count {gene_count} exceeds maximum {MAX_GENES}. "
+            "This may indicate pseudogene contamination or inclusion of non-coding genes."
+        )
+        passed = False
+    else:
+        messages.append(
+            f"Gene count {gene_count} is within expected range ({MIN_GENES}-{MAX_GENES})"
+        )
+
+    # Check all IDs start with ENSG
+    non_ensg = [g for g in genes if not g.startswith('ENSG')]
+    if non_ensg:
+        messages.append(
+            f"FAILED: Found {len(non_ensg)} gene IDs not in ENSG format "
+            f"(examples: {non_ensg[:5]})"
+        )
+        passed = False
+    else:
+        messages.append("All gene IDs are in ENSG format")
+
+    # Check for duplicates
+    unique_genes = set(genes)
+    if len(unique_genes) < gene_count:
+        duplicates = gene_count - len(unique_genes)
+        messages.append(
+            f"FAILED: Found {duplicates} duplicate gene IDs"
+        )
+        passed = False
+    else:
+        messages.append("No duplicate gene IDs found")
+
+    logger.info(
+        f"Gene universe validation: {'PASSED' if passed else 'FAILED'} "
+        f"({gene_count} genes)"
+    )
+
+    return ValidationResult(
+        passed=passed,
+        messages=messages,
+    )
diff --git a/tests/test_gene_mapping.py b/tests/test_gene_mapping.py
new file mode 100644
index 0000000..ab7e63b
--- /dev/null
+++ b/tests/test_gene_mapping.py
@@ -0,0 +1,328 @@
+"""Tests for gene ID mapping module.
+
+Tests gene universe retrieval, batch mapping, and validation gates.
+Uses mocked mygene responses to avoid real API calls.
+"""
+
+from pathlib import Path
+from unittest.mock import MagicMock, patch
+
+import pytest
+
+from usher_pipeline.gene_mapping import (
+    GeneMapper,
+    MappingReport,
+    MappingResult,
+    MappingValidator,
+    ValidationResult,
+    validate_gene_universe,
+)
+
+
+# Mock mygene response fixtures
+
+MOCK_SUCCESSFUL_RESPONSE = {
+    'out': [
+        {
+            'query': 'ENSG00000139618',
+            'symbol': 'BRCA2',
+            'uniprot': {'Swiss-Prot': 'P51587'}
+        },
+        {
+            'query': 'ENSG00000141510',
+            'symbol': 'TP53',
+            'uniprot': {'Swiss-Prot': 'P04637'}
+        },
+        {
+            'query': 'ENSG00000012048',
+            'symbol': 'BRCA1',
+            'uniprot': {'Swiss-Prot': 'P38398'}
+        },
+    ],
+    'missing': []
+}
+
+MOCK_RESPONSE_WITH_NOTFOUND = {
+    'out': [
+        {
+            'query': 'ENSG00000139618',
+            'symbol': 'BRCA2',
+            'uniprot': {'Swiss-Prot': 'P51587'}
+        },
+        {
+            'query': 'ENSG00000141510',
+            'symbol': 'TP53',
+            'uniprot': {'Swiss-Prot': 'P04637'}
+        },
+        {
+            'query': 'ENSG00000000000',
+            'notfound': True,
+        },
+    ],
+    'missing': ['ENSG00000000000']
+}
+
+MOCK_RESPONSE_WITH_UNIPROT_LIST = {
+    'out': [
+        {
+            'query': 'ENSG00000139618',
+            'symbol': 'BRCA2',
+            'uniprot': {'Swiss-Prot': ['P51587', 'Q9UBX7']}  # List of accessions
+        },
+    ],
+    'missing': []
+}
+
+
+# Test MappingResult creation
+
+def test_mapping_result_creation():
+    """Test creating MappingResult with all fields."""
+    result = MappingResult(
+        ensembl_id='ENSG00000139618',
+        hgnc_symbol='BRCA2',
+        uniprot_accession='P51587',
+        mapping_source='mygene'
+    )
+
+    assert result.ensembl_id == 'ENSG00000139618'
+    assert result.hgnc_symbol == 'BRCA2'
+    assert result.uniprot_accession == 'P51587'
+    assert result.mapping_source == 'mygene'
+
+
+def test_mapping_result_with_none_values():
+    """Test MappingResult handles missing data."""
+    result = MappingResult(
+        ensembl_id='ENSG00000000000',
+    )
+
+    assert result.ensembl_id == 'ENSG00000000000'
+    assert result.hgnc_symbol is None
+    assert result.uniprot_accession is None
+    assert result.mapping_source == 'mygene'
+
+
+# Test GeneMapper with mocked mygene
+
+def test_mapper_handles_successful_mapping():
+    """Test mapper with all genes successfully mapped."""
+    with patch('mygene.MyGeneInfo') as mock_mygene:
+        mock_mg = MagicMock()
+        mock_mg.querymany.return_value = MOCK_SUCCESSFUL_RESPONSE
+        mock_mygene.return_value = mock_mg
+
+        mapper = GeneMapper(batch_size=1000)
+        results, report = mapper.map_ensembl_ids([
+            'ENSG00000139618',
+            'ENSG00000141510',
+            'ENSG00000012048',
+        ])
+
+        # Check results
+        assert len(results) == 3
+        assert results[0].ensembl_id == 'ENSG00000139618'
+        assert results[0].hgnc_symbol == 'BRCA2'
+        assert results[0].uniprot_accession == 'P51587'
+
+        # Check report
+        assert report.total_genes == 3
+        assert report.mapped_hgnc == 3
+        assert report.mapped_uniprot == 3
+        assert report.success_rate_hgnc == 1.0
+        assert report.success_rate_uniprot == 1.0
+        assert len(report.unmapped_ids) == 0
+
+
+def test_mapper_handles_unmapped_genes():
+    """Test mapper with one gene not found."""
+    with patch('mygene.MyGeneInfo') as mock_mygene:
+        mock_mg = MagicMock()
+        mock_mg.querymany.return_value = MOCK_RESPONSE_WITH_NOTFOUND
+        mock_mygene.return_value = mock_mg
+
+        mapper = GeneMapper()
+        results, report = mapper.map_ensembl_ids([
+            'ENSG00000139618',
+            'ENSG00000141510',
+            'ENSG00000000000',
+        ])
+
+        # Check results
+        assert len(results) == 3
+        assert results[2].ensembl_id == 'ENSG00000000000'
+        assert results[2].hgnc_symbol is None
+        assert results[2].uniprot_accession is None
+
+        # Check report
+        assert report.total_genes == 3
+        assert report.mapped_hgnc == 2
+        assert report.mapped_uniprot == 2
+        assert abs(report.success_rate_hgnc - 0.667) < 0.01
+        assert abs(report.success_rate_uniprot - 0.667) < 0.01
+        assert 'ENSG00000000000' in report.unmapped_ids
+        assert len(report.unmapped_ids) == 1
+
+
+def test_mapper_handles_uniprot_list():
+    """Test mapper handles UniProt Swiss-Prot as list (takes first)."""
+    with patch('mygene.MyGeneInfo') as mock_mygene:
+        mock_mg = MagicMock()
+        mock_mg.querymany.return_value = MOCK_RESPONSE_WITH_UNIPROT_LIST
+        mock_mygene.return_value = mock_mg
+
+        mapper = GeneMapper()
+        results, report = mapper.map_ensembl_ids(['ENSG00000139618'])
+
+        # Should take first UniProt accession from list
+        assert results[0].uniprot_accession == 'P51587'
+        assert report.mapped_uniprot == 1
+
+
+def test_mapper_batching():
+    """Test mapper processes genes in batches."""
+    with patch('mygene.MyGeneInfo') as mock_mygene:
+        mock_mg = MagicMock()
+        # Return empty response for each batch
+        mock_mg.querymany.return_value = {'out': [], 'missing': []}
+        mock_mygene.return_value = mock_mg
+
+        mapper = GeneMapper(batch_size=2)
+        # 5 genes should result in 3 batches (2+2+1)
+        gene_ids = [f'ENSG{i:011d}' for i in range(5)]
+        results, report = mapper.map_ensembl_ids(gene_ids)
+
+        # Check querymany was called 3 times (3 batches)
+        assert mock_mg.querymany.call_count == 3
+
+
+# Test MappingValidator
+
+def test_validator_passes_high_rate():
+    """Test validator passes with success rate above minimum."""
+    report = MappingReport(
+        total_genes=100,
+        mapped_hgnc=95,
+        mapped_uniprot=90,
+        unmapped_ids=[f'ENSG{i:011d}' for i in range(5)],
+    )
+
+    validator = MappingValidator(min_success_rate=0.90)
+    result = validator.validate(report)
+
+    assert result.passed is True
+    assert result.hgnc_rate == 0.95
+    assert result.uniprot_rate == 0.90
+    assert any('PASSED' in msg for msg in result.messages)
+
+
+def test_validator_fails_low_rate():
+    """Test validator fails with success rate below minimum."""
+    report = MappingReport(
+        total_genes=100,
+        mapped_hgnc=80,
+        mapped_uniprot=75,
+        unmapped_ids=[f'ENSG{i:011d}' for i in range(20)],
+    )
+
+    validator = MappingValidator(min_success_rate=0.90)
+    result = validator.validate(report)
+
+    assert result.passed is False
+    assert result.hgnc_rate == 0.80
+    assert any('FAILED' in msg for msg in result.messages)
+
+
+def test_validator_warns_medium_rate():
+    """Test validator passes with warning for medium success rate."""
+    report = MappingReport(
+        total_genes=100,
+        mapped_hgnc=92,
+        mapped_uniprot=88,
+        unmapped_ids=[f'ENSG{i:011d}' for i in range(8)],
+    )
+
+    validator = MappingValidator(min_success_rate=0.90, warn_threshold=0.95)
+    result = validator.validate(report)
+
+    # Should pass but with warning
+    assert result.passed is True
+    assert result.hgnc_rate == 0.92
+    assert any('WARNING' in msg for msg in result.messages)
+
+
+def test_save_unmapped_report(tmp_path):
+    """Test saving unmapped gene IDs to file."""
+    report = MappingReport(
+        total_genes=100,
+        mapped_hgnc=95,
+        mapped_uniprot=90,
+        unmapped_ids=['ENSG00000000001', 'ENSG00000000002', 'ENSG00000000003'],
+    )
+
+    validator = MappingValidator()
+    output_path = tmp_path / "unmapped_genes.txt"
+    validator.save_unmapped_report(report, output_path)
+
+    # Check file was created and contains expected content
+    assert output_path.exists()
+    content = output_path.read_text()
+    assert '# Unmapped Gene IDs' in content
+    assert '# Total unmapped: 3' in content
+    assert 'ENSG00000000001' in content
+    assert 'ENSG00000000002' in content
+    assert 'ENSG00000000003' in content
+
+
+# Test validate_gene_universe
+
+def test_validate_gene_universe_valid():
+    """Test gene universe validation with valid data."""
+    genes = [f'ENSG{i:011d}' for i in range(20000)]  # 20k genes
+    result = validate_gene_universe(genes)
+
+    assert result.passed is True
+    assert any('within expected range' in msg for msg in result.messages)
+    assert any('ENSG format' in msg for msg in result.messages)
+    assert any('No duplicate' in msg for msg in result.messages)
+
+
+def test_validate_gene_universe_invalid_count():
+    """Test gene universe validation fails with too many genes."""
+    genes = [f'ENSG{i:011d}' for i in range(50000)]  # 50k genes (too many)
+    result = validate_gene_universe(genes)
+
+    assert result.passed is False
+    assert any('exceeds maximum' in msg for msg in result.messages)
+
+
+def test_validate_gene_universe_invalid_format():
+    """Test gene universe validation fails with non-ENSG IDs."""
+    genes = [f'ENSG{i:011d}' for i in range(19500)]
+    genes.extend(['INVALID001', 'INVALID002'])  # Add invalid IDs
+
+    result = validate_gene_universe(genes)
+
+    assert result.passed is False
+    assert any('not in ENSG format' in msg for msg in result.messages)
+
+
+def test_validate_gene_universe_duplicates():
+    """Test gene universe validation fails with duplicates."""
+    genes = [f'ENSG{i:011d}' for i in range(19500)]
+    genes.extend(['ENSG00000000001', 'ENSG00000000002'])  # Add duplicates
+
+    result = validate_gene_universe(genes)
+
+    assert result.passed is False
+    assert any('duplicate' in msg for msg in result.messages)
+
+
+def test_validate_gene_universe_too_few():
+    """Test gene universe validation fails with too few genes."""
+    genes = [f'ENSG{i:011d}' for i in range(1000)]  # Only 1k genes
+
+    result = validate_gene_universe(genes)
+
+    assert result.passed is False
+    assert any('below minimum' in msg for msg in result.messages)