def map_transcript_id2gene_symbol(maf_df):
    """Create and return a mapping from transcript_id to gene_symbol. Assume that each
    transcript_id uniquely maps to a gene_symbol."""

    transcript_id2gene_symbol = dict()

    for index, row in maf_df.iterrows():

        gene_symbol = row['Gene_Symbol']  # e.g. 'AFF2'
        transcript_id = row['Transcript_ID']  # e.g. 'ENST00000369354' (Ensembl transcript ID)

        if transcript_id not in transcript_id2gene_symbol:
            transcript_id2gene_symbol[transcript_id] = gene_symbol
        else:
            assert transcript_id2gene_symbol[transcript_id] == gene_symbol

    return transcript_id2gene_symbol