Generation of Tr2 and Tr4 null and conditionally mutant mice. Schematic illustrations of the wild-type, targeted, null, floxed, and conditional null alleles of Tr2 (A) and Tr4 (B). The null and floxed alleles of Tr2 or Tr4 were generated by intercrossing the respective gene targeted mice with Cre- or Flp-expressing transgenic lines, respectively. To generate the Tr2 null allele, exons 4 and 5 encoding the DNA binding domain were excised, whereas the Tr4 null allele was deleted for exon 5 encoding part of the DNA binding domain. In both floxed alleles, a DNA fragment containing the rest of the coding exons for each gene and the SV40 polyadenylation sequence was fused in-frame to the respective exons 5. The Tr2 or Tr4 conditional null allele was generated by Cre-mediated excision of the respective floxed allele, leaving behind a fluorescent reporter gene (CFP or YFP) flanked by 1 loxP and 1 Frt site. CFP, cyan fluorescent protein; YFP, yellow fluorescent protein.