SOX2 and neoplasm: While p18 has previously been characterized as a “negative regulator” of cell cycle progression and a suppressor of tumor growth, the results of our current study unexpectedly demonstrate that ectopic expression of p18 can enhance the growth of mouse ES cells concomitant with up-regulation of various embryonic markers (e.g., Oct4, Nanog, Sox2, and Rex1) and down-regulation of various differentiation markers (e.g., Gata6, Map2, Cdx2, and BRACHYURY).