Generation and characterization of JAK2-Ex12 transgenic lines. (A) Map of the human JAK2 BAC carrying the N542-E543del mutation in exon 12 (shown in red). The mutated region is shown enlarged below. In the presence of Cre recombinase, the mutated exon 12 is flipped from the inverse orientation (red) to the correct orientation (green). Recombination generates 1 WT loxP and 1 double-mutant lox66/77 site, which is no longer substrate for Cre recombinase. The DNA and amino acid sequences of the mutant (N542-E543del) and WT JAK2 are shown at the bottom. (B) Transgene copy number in 3 transgenic lines, named N1, N2, and N3. These mice were crossed with the IFN-inducible MxCre strain and analyzed for transgene copy number (n) by RT-PCR before and 24 weeks after induction with pIpC. The average values obtained from 3 mice per group are shown with error bars indicating ± SEM. (C) Peripheral blood parameters in MxCre;N1, MxCre;N2, and MxCre;N3 double transgenic mice. Blood counts were determined 16 weeks after 1× pIpC injection. Horizontal lines represent the average values. The group sizes were: n = 9 per double transgenic strain and n = 8 for the WT controls. One-way ANOVA with subsequent Bonferroni post-test was used. *P < .05. cDNA, complementary DNA; IFN, interferon; Hb, hemoglobin; pA, polyadenylation signal from SV40; Plt, platelet; SA, splice acceptor; SD, splice donor.