ES cells were transfected with a construct containing exons 4 and 5 flanked by loxP sites. The construct also contained an IRES-lacZ-pA cassette fused to the 5' end of exon 4 and a neomycin resistance cassette surrounded by Frt sites. This allele was generated by crossing mice carrying the floxed gene with mice carrying PGK-Cre and and a second strain of mice carrying the Flp transgene. Western blot analysis of E12.5 embryos did not detect protein. (J:92665)