def ref_seq_nt_output(in_seq_name, in_ref_name, nt, ext):
    """
    Generate output file name
    :param in_seq_name: treatment name (str)
    :param in_ref_name: reference name (str)
    :param nt: aligned read length (int)
    :param ext: extension (ie. csv or pdf)
    :return: output filename (str)
    """
    return '{0}_{1}_{2}.{3}'.format(in_ref_name, in_seq_name, str(nt), ext)
