The sequenced bases in the alignment that align to this locus in the
        genome, as a string.

        Empty string in the case of a deletion. String of length > 1 if there
        is an insertion here.

def bases(self):
        '''
        The sequenced bases in the alignment that align to this locus in the
        genome, as a string.

        Empty string in the case of a deletion. String of length > 1 if there
        is an insertion here.
        '''
        sequence = self.alignment.query_sequence
        assert self.offset_end <= len(sequence), \
            "End offset=%d > sequence length=%d. CIGAR=%s. SEQUENCE=%s" % (
                self.offset_end,
                len(sequence),
                self.alignment.cigarstring,
                sequence)
        return sequence[self.offset_start:self.offset_end]