def _create_transcript_index(subexon_table):
    """Return a pandas' DataFrame with the gene and transcript ids."""
    transcript_id_columns = ['GeneID', 'TranscriptID']
    unique_transcripts = subexon_table.drop_duplicates(subset=
        transcript_id_columns)
    unique_transcripts = unique_transcripts.loc[:, (transcript_id_columns)]
    unique_transcripts = unique_transcripts.assign(TranscriptIndex=list(
        range(0, unique_transcripts.shape[0])))
    output = subexon_table.merge(unique_transcripts)
    return output
