Mb1-CreΔPB mice develop B-ALL. (A) Mb1-CreΔPB mice (ΔPB) mice developed B-ALL characterized by splenic and thymic enlargement (indicated by arrows). (B) Percentage survival of mice of the indicated genotypes: Mb1^+/CreSpi1^lox/loxSpib^−/− (Mb1-CreΔPB; n = 43); Mb1^+/CreSpi1^+/+Spib^−/− mice (Mb1-CreΔB; n = 36) and Mb1^+/+Spi1^lox/loxSpib^−/− (Mb1-CreΔB; n = 14). (C) Comparisons of enlarged spleens and thymuses extracted from Mb1-CreΔPB mice compared with control ΔB mice (Mb1^+/+Spi1^lox/loxSpib^−/−). (D) Spleen (left) and thymus (right) weight in grams relative to the body weight in WT and Mb1-CreΔPB mice. WT, n = 10 (spleen), 5 (thymus); Mb1-CreΔPB, n = 11 (spleen), 8 (thymus). Significance was determined using unpaired Student t test. *P ≤ .05. (E). Histologic sections (hematoxylin and eosin staining) of spleen and thymus illustrating the lymphocytic infiltration and loss of organs normal structure in Mb1-CreΔPB compared with controls Mb1ΔB (magnification ×4; scale bar represents 200 μm).