IL-7R, FLT3, and CD43 are surface antigens expressed during the transition from pro-B to pre-B cells in BM. To understand interactions between their signaling pathways, we analyzed spontaneous mouse B-LBLs with dual MLV integration into Stat5a and Fiz1 or Stat5a and Hipk2. MLV integration resulted in up-regulation of these genes in lymphoma cells compared with normal pro-B cells from the BM. In lymphomas with both integrations into Stat5a and Fiz1, increases in phosphorylated STAT5A and expression of c-Myc, a target gene of STAT5A, were observed following stimulation of the FLT3. Clones with the dual integrations grew faster in IL-7 and FLT3L-supplemented medium than clones with Stat5a integration alone. On the other hand, in lymphomas with integrations into Stat5a and Hipk2, increases in phosphorylated STAT5A and expression of c-Myc were observed following cross- linking of CD43. In conclusion, FLT3 and CD43 signaling pathways involve STAT5A via Fiz1 and Hipk2 in B-LBLs. Identification of the dual MLV integration sites in B-LBLs, therefore, will provide an excellent tool for identification of the signaling pathways in B-LBLs. J. Leukoc. Biol. 88: 107-116; 2010.