Blood leukocyte values and survival in Mx1-Cre, Nf1 ^flox/flox and Nf1 ^flox/flox mice. (A) PCR analysis of leukocyte DNA from 6-week-old pups that received a single injection of pI-pC shortly after birth. PCR amplification of the unrearranged Nf1^flox allele yields a 350-bp product. A 280-bp fragment corresponding to a deletion of exon 31 (Δ31) is visible in 2 pups that inherited the Mx1-Cre transgene (+) but not in 3 pups that did not (-). Absence of the unrearranged allele in lanes 3 and 4 confirms a high efficiency of somatic recombination. (B) White blood cell counts (WBCs) in 3-month-old pI-pC-treated Mx1-Cre, Nf1^flox/flox (Cre⁺)(n = 21) and control Nf1^flox/flox littermates that did not inherit the Mx1-Cre transgene (Cre^-) (n = 18). The abbreviations are LY, lymphocytes; NE, neutrophils; MO, monocytes; APC, absolute phagocyte count (neutrophils + monocytes). Leukocyte counts are expressed as ± SEM. Asterisks indicate significant differences (P < .05 by Student t test) between the Cre⁺ and Cre^- animals. (C) A composite photomicrograph (original magnification × 400) of peripheral blood from a Cre⁺ mouse shows mature neutrophils (top left), intermediate forms (top right), a monocyte and a mature neutrophil (bottom left), and an intermediate form, which is likely in the monocytic lineage (bottom right). (D) Kaplan-Meier analysis demonstrates a significant reduction in survival in Cre⁺ (n = 59) versus Cre^- (n = 72) littermates (P < .0001).