Myb overexpression enables the formation of B-cell and myeloid neoplasms in vivo. (A) Schematic representation of R26-Myb mice that allow Cre-dependent conditional expression of a bicistronic transgene transcript, encoding for Myb and the eGFP/Luciferase reporter, from the Rosa26 promoter. Breeding scheme to obtain R26-Myb^tg/tg;VaviCre^tg/+ (Myb^Vav) mice with hematopoietic-specific overexpression of Myb. R26, Rosa26; IRES, independent ribosomal entry site; eGFP, enhanced green fluorescent protein; PKG, phosphoglycerate kinase 1; Neo^R, neomycin resistance gene. (B) Kaplan Meier survival curve of Cre-negative control (R26-Myb^tg/tg) vs Myb^Vav mice. A log-rank Mantel-Cox test showed that survival of Myb^Vav mice was significantly lower. *P = .0302. (C) Peripheral blood values of Myb^vav mice and nonrecombined controls. WBC, white blood cells; RBC, red blood cells. An unpaired t-test indicated that there was no significant difference between tumor-carrying Myb^vav mice and nonrecombined controls. (D) Graph depicting the spleen-to-body weight ratio of Myb^vav mice that developed neoplasm and age-matched Cre-negative littermate control mice. *P = .0450. (E) Flow cytometry analysis of 4 Myb^Vav tumors. Single live cells were analyzed for the T-cell marker Thy1.1 (CD90), B-cell markers B220 and CD19, and myeloid markers Gr-1 and Cd11b. FSC-A, forward scatter area. (F) Left: heatmap summarizing flow data of tumor samples, including BM, LN, and spleen, from 4 Myb^vav mice. Right: Graph depicting the percentage of T cells, B cells, or myeloid cells from panel E, which were pregated for single live cells. (G,H) Hematoxylin and eosin (H&E) staining or representative immunohistochemistry (IHC) for the proliferation marker KI67 or MYB on paraffin sections of Myb^vav liver (G) and LN (H) tumors and of an aged-matched Cre-negative littermate control. Scale bar: 25 µm. Scale bar inset: 50 µm.