CZ CELLxGENE Census

Overview

The CZ CELLxGENE Census provides programmatic access to a comprehensive, versioned collection of standardized single-cell and spatial transcriptomics data from CZ CELLxGENE Discover. This skill enables efficient querying and analysis of public Census releases without downloading whole datasets first.

The Census includes:

• 217+ million total cells and 125+ million unique cells in the 2025-11-08 stable LTS release
• 1,845 datasets in the 2025-11-08 stable LTS release
• Human, mouse, marmoset, rhesus macaque, and chimpanzee data in the current schema
• Standardized metadata (cell types, tissues, diseases, donors)
• Raw gene expression matrices and source H5AD lookup/download helpers
• Pre-calculated summary counts, embeddings, and spatial data
• Integration with AnnData, Scanpy, TileDB-SOMA, TileDB-SOMA-ML, and other analysis tools

When to Use This Skill

This skill should be used when:

• Querying single-cell expression data by cell type, tissue, or disease
• Exploring available single-cell datasets and metadata
• Training machine learning models on single-cell data
• Performing large-scale cross-dataset analyses
• Integrating Census data with scanpy or other analysis frameworks
• Computing statistics across millions of cells
• Accessing pre-calculated embeddings or model predictions

Installation and Setup

Install the Census API:

uv pip install "cellxgene-census==1.18.0"

For spatial workflows:

uv pip install "cellxgene-census[spatial]==1.18.0"

For PyTorch model training, use TileDB-SOMA-ML. The old cellxgene_census.experimental.ml loaders are absent from 1.18.0:

uv pip install "cellxgene-census==1.18.0" tiledbsoma-ml

Core Workflow Patterns

Eight patterns, each with code, are in references/core_workflow_patterns.md:

1. Opening the Census — always pin census_version so an analysis stays reproducible.
2. Exploring Census information — available datasets, cell counts, and summary tables.
3. Querying expression data — small to medium scale into an AnnData.
4. Large-scale queries — out-of-core processing when the slice will not fit in memory.
5. Machine learning with PyTorch — TileDB-SOMA-ML data loaders.
6. Spatial Census data — accessing spatial assays.
7. Integration with Scanpy — handing a Census slice to a standard Scanpy workflow.
8. Multi-dataset integration — combining datasets and handling batch effects.

Key Concepts and Best Practices

The examples pin the current LTS build 2025-11-08, verified through the live release directory on 2026-09-30. The SDK and data release are separate versions. Resolve stable once with get_census_version_description("stable")["release_build"] and record that date; never silently switch builds midway through analysis.

Always Filter for Primary Data

Unless analyzing duplicates, always include is_primary_data == True in queries to avoid counting cells multiple times:

obs_value_filter="cell_type == 'B cell' and is_primary_data == True"

Specify Census Version for Reproducibility

Always specify the Census version in production analyses:

census = cellxgene_census.open_soma(census_version="2025-11-08")

Estimate Query Size Before Loading

For large queries, count the selected rows without loading every metadata column. Cell count alone is not a memory estimate: gene count, sparsity, dtype, layers, embeddings, and downstream dense copies also matter:

import tiledbsoma as soma
with census["census_data"]["homo_sapiens"].axis_query(
    measurement_name="RNA",
    obs_query=soma.AxisQuery(
        value_filter="tissue_general == 'brain' and is_primary_data == True"
    ),
) as query:
    print(f"Selected {query.n_obs:,} cells and {query.n_vars:,} genes")

# Query axes consume memory too; stream expression if the matrix will not fit.

Use tissue_general for Broader Groupings

The tissue_general field provides coarser categories than tissue, useful for cross-tissue analyses:

# Broader grouping
obs_value_filter="tissue_general == 'immune system'"

# Specific tissue
obs_value_filter="tissue == 'venous blood'"

Select Only Needed Columns

Minimize data transfer by specifying only required metadata columns:

obs_column_names=["cell_type", "tissue_general", "disease"]  # Not all columns

Check Dataset Presence for Gene-Specific Queries

When analyzing specific genes, verify which datasets measured them:

genes = cellxgene_census.get_var(
    census, "homo_sapiens",
    value_filter="feature_name in ['CD4', 'CD8A']",
    column_names=["soma_joinid", "feature_id", "feature_name"],
)
presence = cellxgene_census.get_presence_matrix(census, "homo_sapiens")
# Columns use Census join IDs, not positions in the filtered gene table.
gene_presence = presence[:, genes["soma_joinid"].to_numpy()]

Gene symbols are not necessarily unique in schema 2.4.0; keep feature_id as the feature key, inspect all symbol matches, and never silently select the first match.

Presence rows are dataset soma_joinid values, not cell IDs; a zero means the feature was not measured in that dataset, not that measured expression was zero. The remote-query snippets are illustrative; verify the selected release and returned schema before loading a large expression slice.

Two-Step Workflow: Explore Then Query

First explore metadata to understand available data, then query expression:

# Step 1: Explore what's available
metadata = cellxgene_census.get_obs(
    census, "homo_sapiens",
    value_filter="disease == 'COVID-19' and is_primary_data == True",
    column_names=["cell_type", "tissue_general"]
)
print(metadata.value_counts())

# Step 2: Query based on findings
adata = cellxgene_census.get_anndata(
    census=census,
    organism="Homo sapiens",
    obs_value_filter="disease == 'COVID-19' and cell_type == 'T cell' and is_primary_data == True",
)

For complete disease cohorts, use the multi-value disease workflow in references/common_patterns.md. Exact equality in the small examples selects only cells whose whole disease field equals that label.

Available Metadata Fields

Cell Metadata (obs)

Key fields for filtering:

• cell_type, cell_type_ontology_term_id
• tissue, tissue_general, tissue_ontology_term_id
• disease, disease_ontology_term_id
• assay, assay_ontology_term_id
• donor_id, sex, self_reported_ethnicity
• development_stage, development_stage_ontology_term_id
• dataset_id
• is_primary_data (Boolean: True = primary representation)

The current schema includes organism collections beyond human and mouse. Confirm available organisms for the selected release with list(census["census_data"].keys()).

Gene Metadata (var)

• feature_id (Ensembl gene ID, e.g., "ENSG00000161798")
• feature_name (Gene symbol, e.g., "FOXP2")
• feature_type (present in the verified 2025-11-08 build; inspect the selected schema)
• feature_length (Gene length in base pairs)
• nnz, n_measured_obs (availability summaries useful for checking sparsity and coverage)

Reference Documentation

This skill includes detailed reference documentation:

Sources, endpoint contracts, H5AD lookup, and embeddings are documented in references/api_access.md.

references/census_schema.md

Comprehensive documentation of:

• Census data structure and organization
• All available metadata fields
• Value filter syntax and operators
• SOMA object types
• Data inclusion criteria

When to read: When you need detailed schema information, full list of metadata fields, or complex filter syntax.

references/common_patterns.md

Examples and patterns for:

• Exploratory queries (metadata only)
• Small-to-medium queries (AnnData)
• Large queries (out-of-core processing)
• PyTorch integration
• Spatial Census access patterns
• Scanpy integration workflows
• Multi-dataset integration
• Best practices and common pitfalls

When to read: When implementing specific query patterns, looking for code examples, or troubleshooting common issues.

Common Use Cases

Use Case 1: Explore Cell Types in a Tissue

with cellxgene_census.open_soma(census_version="2025-11-08") as census:
    cells = cellxgene_census.get_obs(
        census, "homo_sapiens",
        value_filter="tissue_general == 'lung' and is_primary_data == True",
        column_names=["cell_type"]
    )
    print(cells["cell_type"].value_counts())

Use Case 2: Query Marker Gene Expression

with cellxgene_census.open_soma(census_version="2025-11-08") as census:
    adata = cellxgene_census.get_anndata(
        census=census,
        organism="Homo sapiens",
        var_value_filter="feature_name in ['CD4', 'CD8A', 'CD19']",
        obs_value_filter="cell_type in ['T cell', 'B cell'] and is_primary_data == True",
    )

Use Case 3: Read Cell Type Classifier Batches

import tiledbsoma as soma
from tiledbsoma_ml import ExperimentDataset, experiment_dataloader

with cellxgene_census.open_soma(census_version="2025-11-08") as census:
    experiment = census["census_data"]["homo_sapiens"]
    with experiment.axis_query(
        measurement_name="RNA",
        obs_query=soma.AxisQuery(value_filter="is_primary_data == True"),
    ) as query:
        dataset = ExperimentDataset(
            query=query,
            layer_name="raw",
            obs_column_names=["cell_type"],
            batch_size=128,
            shuffle=True,
        )
        dataloader = experiment_dataloader(dataset)

        for X, obs in dataloader:
            labels = obs["cell_type"]
            # Training logic
            pass

Use Case 4: Cross-Tissue Analysis

with cellxgene_census.open_soma(census_version="2025-11-08") as census:
    adata = cellxgene_census.get_anndata(
        census=census,
        organism="Homo sapiens",
        obs_value_filter="cell_type == 'macrophage' and tissue_general in ['lung', 'liver', 'brain'] and is_primary_data == True",
    )

    # Exploratory cell-level marker ranking; Census X contains raw counts.
    import scanpy as sc
    adata.layers["counts"] = adata.X.copy()
    sc.pp.normalize_total(adata, target_sum=1e4)
    sc.pp.log1p(adata)
    sc.tl.rank_genes_groups(adata, groupby="tissue_general")

For tissue-effect inference, aggregate or model biological replicates using donor and study provenance. Thousands of cells from one donor are not thousands of independent replicates, and tissue effects can be confounded with dataset or assay.

Troubleshooting

Query Returns Too Many Cells

• Add more specific filters to reduce scope
• Use tissue instead of tissue_general for finer granularity
• Filter by specific dataset_id if known
• Switch to out-of-core processing for large queries

Memory Errors

• Reduce query scope with more restrictive filters
• Select fewer genes with var_value_filter
• Use out-of-core processing with axis_query()
• Process data in batches

Duplicate Cells in Results

• Always include is_primary_data == True in filters
• Check if intentionally querying across multiple datasets

Gene Not Found

• Verify gene name spelling (case-sensitive)
• Try Ensembl ID with feature_id instead of feature_name
• Check dataset presence matrix to see if gene was measured
• Some genes may have been filtered during Census construction

Version Inconsistencies

• Always specify census_version explicitly
• Use same version across all analyses
• Check release notes for version-specific changes