def get_range_upstream(pam_pos, guide_len):
    """
    Get positions N bp upstream, i.e. for forward 3' PAMs or reverse 5' PAMs
    :param pam_pos: position of PAM, int.
    :return: sgRNA seed region positions, set of ints.
    """
    sgrna = set(range(pam_pos - (guide_len + 1), pam_pos))
    return sgrna