def check_aggregator(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.')
