A loxP site was inserted upstream of exon 5 and another downstream of exon 7; a PGK-neo/HSV-tk cassette (neo/tk) followed by a third loxP site was inserted in intron 7 further downstream. The present allele was generated by Cre recombinase excision of the neo/tk cassette and exons 5 through 7 of the endogenous gene. (J:159441)