def _is_gse1(filename):
    """
    Checks whether a file is GSE1 or not.

    :type filename: str
    :param filename: GSE1 file to be checked.
    :rtype: bool
    :return: ``True`` if a GSE1 file.
    """
    # Open file.
    with open(filename, 'rb') as f:
        try:
            data = f.readline()
        except Exception:
            return False
    if data.startswith(b'WID1') or data.startswith(b'XW01'):
        return True
    return False