def METHOD_NAME(aggregator, source, expression_type, group_by):
    """Check aggregator fields."""
    if aggregator["source"] != source:
        raise ValueError(
            "All expressions must be annotated by the same genome database (NCBI, UCSC, ENSEMBLE,...)."
        )
    if aggregator["expression_type"] != expression_type:
        raise ValueError("All expressions must be of the same type.")
    if aggregator["group_by"] != group_by:
        raise ValueError("Group by field must be the same.")