To understand the basis for T-ALL development in the R26PR;MMTV-cre line and to characterize the cellular changes taking place prior to disease development, we mated MMTV-cre mice to a Cre reporter line that permanently tags Cre-expressing cells with GFP (mTmG) (Muzumdar et al., 2007) and compared the GFP+ and GFP− hematopoietic progenitor populations in bone marrow of WT, mTmG;MMTV-cre, R26PR;Mx1-cre and R26PR;MMTV-cre animals approximately 2 weeks prior to disease onset (Fig. 4A–F). Here, MX1 is linked to acute lymphoblastic leukemia.