diff --git a/pyproject.toml b/pyproject.toml
index f0eaeae..02eabaf 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -36,6 +36,7 @@ dependencies = [
     "httpx>=0.28",
     "structlog>=25.0",
     "biopython>=1.84",
+    "scipy>=1.14",
 ]
 
 [project.optional-dependencies]
diff --git a/src/usher_pipeline/scoring/quality_control.py b/src/usher_pipeline/scoring/quality_control.py
new file mode 100644
index 0000000..42502d0
--- /dev/null
+++ b/src/usher_pipeline/scoring/quality_control.py
@@ -0,0 +1,413 @@
+"""Quality control checks for scoring results."""
+
+import numpy as np
+import polars as pl
+import structlog
+from scipy.stats import median_abs_deviation
+
+from usher_pipeline.persistence.duckdb_store import PipelineStore
+
+logger = structlog.get_logger(__name__)
+
+# Quality control thresholds
+MISSING_RATE_WARN = 0.5  # 50% missing = warning
+MISSING_RATE_ERROR = 0.8  # 80% missing = error
+MIN_STD_THRESHOLD = 0.01  # std < 0.01 = no variation warning
+OUTLIER_MAD_THRESHOLD = 3.0  # >3 MAD from median = outlier
+
+# Evidence layer configuration
+EVIDENCE_LAYERS = [
+    "gnomad",
+    "expression",
+    "annotation",
+    "localization",
+    "animal_model",
+    "literature"
+]
+
+SCORE_COLUMNS = {
+    "gnomad": "gnomad_score",
+    "expression": "expression_score",
+    "annotation": "annotation_score",
+    "localization": "localization_score",
+    "animal_model": "animal_model_score",
+    "literature": "literature_score",
+}
+
+
+def compute_missing_data_rates(store: PipelineStore) -> dict:
+    """
+    Compute fraction of NULL values per score column in scored_genes.
+
+    Args:
+        store: PipelineStore with scored_genes table
+
+    Returns:
+        Dict with keys:
+        - rates: dict[str, float] - NULL rate per layer (0.0-1.0)
+        - warnings: list[str] - layers with >50% missing
+        - errors: list[str] - layers with >80% missing
+
+    Notes:
+        - Queries scored_genes table
+        - Classifies: rate > 0.8 -> error, rate > 0.5 -> warning
+        - Logs each layer's rate with appropriate level
+    """
+    # Query missing data rates in a single SQL query
+    query = f"""
+    SELECT
+        COUNT(*) AS total,
+        AVG(CASE WHEN {SCORE_COLUMNS['gnomad']} IS NULL THEN 1.0 ELSE 0.0 END) AS gnomad_missing,
+        AVG(CASE WHEN {SCORE_COLUMNS['expression']} IS NULL THEN 1.0 ELSE 0.0 END) AS expression_missing,
+        AVG(CASE WHEN {SCORE_COLUMNS['annotation']} IS NULL THEN 1.0 ELSE 0.0 END) AS annotation_missing,
+        AVG(CASE WHEN {SCORE_COLUMNS['localization']} IS NULL THEN 1.0 ELSE 0.0 END) AS localization_missing,
+        AVG(CASE WHEN {SCORE_COLUMNS['animal_model']} IS NULL THEN 1.0 ELSE 0.0 END) AS animal_model_missing,
+        AVG(CASE WHEN {SCORE_COLUMNS['literature']} IS NULL THEN 1.0 ELSE 0.0 END) AS literature_missing
+    FROM scored_genes
+    """
+
+    result = store.conn.execute(query).fetchone()
+    total_genes = result[0]
+
+    # Build rates dict
+    rates = {
+        "gnomad": result[1],
+        "expression": result[2],
+        "annotation": result[3],
+        "localization": result[4],
+        "animal_model": result[5],
+        "literature": result[6],
+    }
+
+    warnings = []
+    errors = []
+
+    # Classify and log each layer
+    for layer, rate in rates.items():
+        if rate > MISSING_RATE_ERROR:
+            errors.append(f"{layer}: {rate:.1%} missing")
+            logger.error(
+                "missing_data_error",
+                layer=layer,
+                missing_rate=f"{rate:.1%}",
+                threshold=f"{MISSING_RATE_ERROR:.0%}",
+            )
+        elif rate > MISSING_RATE_WARN:
+            warnings.append(f"{layer}: {rate:.1%} missing")
+            logger.warning(
+                "missing_data_warning",
+                layer=layer,
+                missing_rate=f"{rate:.1%}",
+                threshold=f"{MISSING_RATE_WARN:.0%}",
+            )
+        else:
+            logger.info(
+                "missing_data_ok",
+                layer=layer,
+                missing_rate=f"{rate:.1%}",
+            )
+
+    logger.info(
+        "compute_missing_data_rates_complete",
+        total_genes=total_genes,
+        warnings_count=len(warnings),
+        errors_count=len(errors),
+    )
+
+    return {
+        "rates": rates,
+        "warnings": warnings,
+        "errors": errors,
+    }
+
+
+def compute_distribution_stats(store: PipelineStore) -> dict:
+    """
+    Compute distribution statistics per evidence layer.
+
+    Args:
+        store: PipelineStore with scored_genes table
+
+    Returns:
+        Dict with keys:
+        - distributions: dict[str, dict] - stats per layer (mean, median, std, min, max)
+        - warnings: list[str] - layers with anomalies (no variation)
+        - errors: list[str] - layers with out-of-range values
+
+    Notes:
+        - Computes stats only on non-NULL scores
+        - Detects: std < 0.01 -> no variation warning
+        - Detects: min < 0.0 or max > 1.0 -> out of range error
+    """
+    distributions = {}
+    warnings = []
+    errors = []
+
+    for layer in EVIDENCE_LAYERS:
+        col = SCORE_COLUMNS[layer]
+
+        # Query non-NULL scores for this layer
+        query = f"SELECT {col} FROM scored_genes WHERE {col} IS NOT NULL"
+        result = store.conn.execute(query).fetchall()
+
+        if not result:
+            distributions[layer] = None
+            warnings.append(f"{layer}: no data available")
+            logger.warning("distribution_no_data", layer=layer)
+            continue
+
+        # Extract scores as numpy array
+        scores = np.array([row[0] for row in result])
+
+        # Compute statistics
+        stats = {
+            "mean": float(np.mean(scores)),
+            "median": float(np.median(scores)),
+            "std": float(np.std(scores)),
+            "min": float(np.min(scores)),
+            "max": float(np.max(scores)),
+            "count": len(scores),
+        }
+
+        distributions[layer] = stats
+
+        # Detect anomalies
+        if stats["std"] < MIN_STD_THRESHOLD:
+            warnings.append(f"{layer}: no variation (std={stats['std']:.4f})")
+            logger.warning(
+                "distribution_no_variation",
+                layer=layer,
+                std=f"{stats['std']:.4f}",
+                threshold=MIN_STD_THRESHOLD,
+            )
+
+        if stats["min"] < 0.0 or stats["max"] > 1.0:
+            errors.append(
+                f"{layer}: out of range (min={stats['min']:.4f}, max={stats['max']:.4f})"
+            )
+            logger.error(
+                "distribution_out_of_range",
+                layer=layer,
+                min=f"{stats['min']:.4f}",
+                max=f"{stats['max']:.4f}",
+            )
+
+        logger.info(
+            "distribution_stats",
+            layer=layer,
+            mean=f"{stats['mean']:.4f}",
+            median=f"{stats['median']:.4f}",
+            std=f"{stats['std']:.4f}",
+            range=f"[{stats['min']:.4f}, {stats['max']:.4f}]",
+        )
+
+    logger.info(
+        "compute_distribution_stats_complete",
+        layers_processed=len(EVIDENCE_LAYERS),
+        warnings_count=len(warnings),
+        errors_count=len(errors),
+    )
+
+    return {
+        "distributions": distributions,
+        "warnings": warnings,
+        "errors": errors,
+    }
+
+
+def detect_outliers(store: PipelineStore) -> dict:
+    """
+    Detect outlier genes using MAD-based robust detection.
+
+    Uses Median Absolute Deviation (MAD) to identify genes with scores
+    > 3 MAD from the median per evidence layer.
+
+    Args:
+        store: PipelineStore with scored_genes table
+
+    Returns:
+        Dict with keys per layer:
+        - count: int - number of outliers detected
+        - example_genes: list[str] - up to 5 gene symbols
+
+    Notes:
+        - Computes MAD = median(|scores - median(scores)|)
+        - If MAD == 0 (no variation), skip outlier detection
+        - Flags genes where |score - median| > 3 * MAD
+    """
+    outliers = {}
+
+    for layer in EVIDENCE_LAYERS:
+        col = SCORE_COLUMNS[layer]
+
+        # Query gene_symbol and score for non-NULL scores
+        query = f"""
+        SELECT gene_symbol, {col} as score
+        FROM scored_genes
+        WHERE {col} IS NOT NULL
+        """
+        result = store.conn.execute(query).pl()
+
+        if result.height == 0:
+            outliers[layer] = {"count": 0, "example_genes": []}
+            continue
+
+        # Extract scores as numpy array
+        scores = result["score"].to_numpy()
+        median = np.median(scores)
+
+        # Compute MAD using scipy
+        mad = median_abs_deviation(scores, scale="normal")
+
+        # If MAD is zero (no variation), skip outlier detection
+        if mad == 0 or np.isclose(mad, 0):
+            outliers[layer] = {"count": 0, "example_genes": []}
+            logger.info(
+                "outlier_detection_skipped",
+                layer=layer,
+                reason="no_variation",
+                mad=0,
+            )
+            continue
+
+        # Flag outliers: |score - median| > threshold * MAD
+        deviations = np.abs(scores - median)
+        is_outlier = deviations > (OUTLIER_MAD_THRESHOLD * mad)
+
+        # Get outlier genes
+        outlier_genes = result.filter(pl.Series(is_outlier))["gene_symbol"].to_list()
+        outlier_count = len(outlier_genes)
+
+        outliers[layer] = {
+            "count": outlier_count,
+            "example_genes": outlier_genes[:5],  # Limit to 5 examples
+        }
+
+        if outlier_count > 0:
+            logger.info(
+                "outliers_detected",
+                layer=layer,
+                count=outlier_count,
+                mad=f"{mad:.4f}",
+                threshold=OUTLIER_MAD_THRESHOLD,
+                examples=outlier_genes[:5],
+            )
+        else:
+            logger.info(
+                "no_outliers_detected",
+                layer=layer,
+                mad=f"{mad:.4f}",
+            )
+
+    return outliers
+
+
+def run_qc_checks(store: PipelineStore) -> dict:
+    """
+    Orchestrate all quality control checks.
+
+    Runs missing data, distribution stats, outlier detection, and
+    composite score analysis.
+
+    Args:
+        store: PipelineStore with scored_genes table
+
+    Returns:
+        Dict with keys:
+        - missing_data: dict from compute_missing_data_rates()
+        - distributions: dict from compute_distribution_stats()
+        - outliers: dict from detect_outliers()
+        - composite_stats: dict with composite score statistics
+        - warnings: list[str] - combined warnings from all checks
+        - errors: list[str] - combined errors from all checks
+        - passed: bool - True if no errors
+
+    Notes:
+        - Validates scored_genes table exists
+        - Logs final QC summary with pass/fail status
+    """
+    logger.info("run_qc_checks_start")
+
+    # Run all three checks
+    missing_data = compute_missing_data_rates(store)
+    distributions = compute_distribution_stats(store)
+    outliers = detect_outliers(store)
+
+    # Compute composite score statistics
+    query = """
+    SELECT
+        AVG(composite_score) AS mean,
+        MEDIAN(composite_score) AS median,
+        STDDEV(composite_score) AS std,
+        MIN(composite_score) AS min,
+        MAX(composite_score) AS max,
+        PERCENTILE_CONT(0.10) WITHIN GROUP (ORDER BY composite_score) AS p10,
+        PERCENTILE_CONT(0.25) WITHIN GROUP (ORDER BY composite_score) AS p25,
+        PERCENTILE_CONT(0.50) WITHIN GROUP (ORDER BY composite_score) AS p50,
+        PERCENTILE_CONT(0.75) WITHIN GROUP (ORDER BY composite_score) AS p75,
+        PERCENTILE_CONT(0.90) WITHIN GROUP (ORDER BY composite_score) AS p90,
+        COUNT(*) AS total,
+        COUNT(composite_score) AS non_null
+    FROM scored_genes
+    """
+    result = store.conn.execute(query).fetchone()
+
+    composite_stats = {
+        "mean": result[0],
+        "median": result[1],
+        "std": result[2],
+        "min": result[3],
+        "max": result[4],
+        "p10": result[5],
+        "p25": result[6],
+        "p50": result[7],
+        "p75": result[8],
+        "p90": result[9],
+        "total_genes": result[10],
+        "non_null_count": result[11],
+    }
+
+    logger.info(
+        "composite_score_stats",
+        mean=f"{composite_stats['mean']:.4f}" if composite_stats['mean'] else "N/A",
+        median=f"{composite_stats['median']:.4f}" if composite_stats['median'] else "N/A",
+        std=f"{composite_stats['std']:.4f}" if composite_stats['std'] else "N/A",
+        percentiles={
+            "p10": f"{composite_stats['p10']:.4f}" if composite_stats['p10'] else "N/A",
+            "p25": f"{composite_stats['p25']:.4f}" if composite_stats['p25'] else "N/A",
+            "p50": f"{composite_stats['p50']:.4f}" if composite_stats['p50'] else "N/A",
+            "p75": f"{composite_stats['p75']:.4f}" if composite_stats['p75'] else "N/A",
+            "p90": f"{composite_stats['p90']:.4f}" if composite_stats['p90'] else "N/A",
+        },
+        coverage=f"{composite_stats['non_null_count']}/{composite_stats['total_genes']}",
+    )
+
+    # Combine warnings and errors
+    combined_warnings = (
+        missing_data["warnings"] +
+        distributions["warnings"]
+    )
+
+    combined_errors = (
+        missing_data["errors"] +
+        distributions["errors"]
+    )
+
+    passed = len(combined_errors) == 0
+
+    logger.info(
+        "run_qc_checks_complete",
+        total_warnings=len(combined_warnings),
+        total_errors=len(combined_errors),
+        status="PASSED" if passed else "FAILED",
+    )
+
+    return {
+        "missing_data": missing_data,
+        "distributions": distributions,
+        "outliers": outliers,
+        "composite_stats": composite_stats,
+        "warnings": combined_warnings,
+        "errors": combined_errors,
+        "passed": passed,
+    }