Survivin deletion abrogates secondary colony formation induced by ITD-Flt3. Fifty thousand GFP⁺ bone marrow cells transduced with wild-type (WT) Flt3 or ITD-Flt3 (N51) were cultured in soft agar with 30% FBS for 2 weeks in the absence of hematopoietic growth factors. Total colonies were counted and colonies were harvested, mechanically dispersed, and plated in secondary soft agar cultures with 30% FBS but without growth factors in the presence of 1 μM 4OH-tamoxifen. Secondary colonies were scored on day 14. Data are expressed as mean ± SEM of 3 independent experiments. *P < .05 compared with Survivin^flox/flox.