def GetChrom(chrom):
    """
    Extract a numerical chromosome

    Parameters
    ----------
    chrom : str
       Chromosome string

    Returns
    -------
    chrom : int
       Integer-value for the chromosome
       X gets set to 23
    """
    if "X" in chrom: return 23
    if "Y" in chrom: return 24
    if chrom.startswith("chr"):
        return int(chrom[3:])
    else:
        return int(chrom)