Abstract: Described herein are methods and compositions for regulation of the p53 pathway, providing intrinsic “sternness” allowing for their efficient in vitro expansion following isolation. Pharmacologic approaches to modulate p53 signaling supports expansion of stem cells, including greater clonal expansion of lung stem cells when compared to use of other small molecules such as ROCK inhibitor Y27632 alone. Effects of combined treatment with Pifithrin-? and Y27632 are additive. The current invention involves use of drugs that target the p53 pathway to reversibly regulate stem cell expansion in vitro for banking of stem cells and for pre-conditioning of stem cells prior to orthotopic transplantation.