BAC clone CH321-18I14 (encompasses sna and Tim17b2) in which the sna coding sequence has been replaced with sequence encoding the y transcription unit and an approximately 1140bp kanamycin resistance cassette. The sna 'shadow enhancer', a cluster of dl and twi protein binding sites located within the neighbouring Tim17b2 locus, has been replaced by random DNA. The primary sna enhancer (cluster of dl and twi protein binding sites) in the immediate 5' flanking region of the sna transcription unit is still present.