def get_gene_substitution_language() -> ParserElement:
    """Build a gene substitution parser."""
    parser_element = gsub_tag + nest(
        dna_nucleotide(GSUB_REFERENCE),
        ppc.integer(GSUB_POSITION),
        dna_nucleotide(GSUB_VARIANT),
    )
    parser_element.setParseAction(_handle_gsub)
    return parser_element
Build a gene substitution parser.