def valid_chrom(arg):
    """Check if a string represents a valid chromosome"""
    arg = arg.lower()
    if arg[:3] == 'chr':
        return True
    if arg.isdigit() or arg in ('x', 'y', 'mt'):
        return True
    if arg[:8] == 'scaffold' or arg[:2] == 'gl' or arg[:3] == 'par':
        return True
    return False