def normalize_variant(variant: str) -> str:
    """
    Normalize variant.

    Reformat variant replace colons as separators
    to underscore.

    chromosome:position:reference:alternative
    to
    chromosome_position_reference_alternative

    :param variant: string representation of variant
    :return: reformatted variant
    """
    cpra = variant.split(":")
    cpra[0] = "X" if cpra[0] == "23" else cpra[0]
    return "_".join(cpra)