The targeting vector contains the 2924 bp loxTB (loxP-flanked transcriptional blocker) cassette inserted 421 bp into the first intron of the gene. The loxTB cassette contains a loxP-flanked neo cassette consisting of an SV40 enhancer, the neo gene, the HSV-TK pA sequence, and a transcriptional pause signal. (J:253437)