def find_identifier(qualifiers):
    """Finds an identifier from a dictionary of feature qualifiers.

    This function selects for the following fields in decreasing order:
    protein_id, locus_tag, ID and Gene. This should cover most cases where CDS
    features do not have protein ID's.

    Args:
        qualifiers (dict): Feature qualifiers parsed with genome2json.
    Returns:
        Identifier, if found, otherwise None.
    """
    for field in ("protein_id", "locus_tag", "ID", "Gene"):
        try:
            return qualifiers[field]
        except KeyError:
            pass
    return None