def transcript_id_of_protein_id(self, protein_id):
        """
        What is the transcript ID associated with a given protein ID?
        """
        results = self._query_transcript_ids(
            "protein_id",
            protein_id,
            feature="CDS")
        if len(results) == 0:
            raise ValueError("Protein ID not found: %s" % protein_id)
        assert len(results) == 1, \
            ("Should have only one transcript ID for a given protein ID, "
             "but found %d: %s" % (len(results), results))
        return results[0]
What is the transcript ID associated with a given protein ID?