A loxP site was inserted in intron 1b followed by a splice acceptor, SV40 T antigen, IRESEGFP/neo and aCMAZ transcriptional pause. This was followed immediately by a floxed blasticidin-S deaminase sequence. The blasticidin selection cassette was excised from properly targeted ES cells. Very low levels of protein can still be detected in homozygous mice (hypomorphic).