def write_orf_fasta(orf_df, file_out):
    """
    Write ORF sequences to a fasta file.

    Parameters
    ----------
    orf_df : DataFrame
        orf_df DataFrame
    file_out : str
        path to file to write fasta sequences

    """
    orf_df.to_csv(file_out, mode='a', index=False, sep='\n', header=False,
        columns=['fasta_id', 'orf_sequence'])
