To directly assess the role of SIRT1 loss as a leukemia-initiating event in T-ALL transformation, we crossed a mouse line containing floxed alleles for SIRT1 exon 4 with a Mx1-Cre transgenic mouse line to delete SIRT1 exon 4 (Fig. S4a-d). Here, SIRT1 is linked to acute lymphoblastic leukemia.