Integrations of 65 mice identified 12 CIS, the most commonly mutated genes suggesting three major mechanisms for STAT5 mediated transformation: disruption of B-cell development (Sos1, Kdm2a, Ikzf1, Klf3), enhanced JAK/STAT5 signaling (Jak1) and modification of the CDKN2A tumor-suppressor pathway (Bmi1, Cdkn2a). This evidence concerns the gene STAT5B and in situ carcinoma.