Moreover, a subset of TSCs have been reported to express high levels of ES cell marker genes, including Oct4 and Nanog, which have been associated with cancer resistance and 4-Acetyl-1,1-dimethylpiperazinium iodide relapse. Although similarities between ES cells and TSCs may provide a new opportunity to further understand the tumorigenic process, the tumorigenic potential of ES cells also represents a significant hurdle for their therapeutic applications. Thus, defining molecular targets that allow stemness to be dissociated from tumorigenesis is an important goal in ES cell biology, as well as tumor cell biology. Stem cells constantly face the choices of self-renewal, differentiation, migration, quiescence and cell death. Cell cycle regulation is one of the fundamental processes modulating cell fate choices and it represents a unique angle to dissect the relationship between tumorigenesis and stemness. p18, an INK4 family member, A 331440 dihydrochloride suppresses CDK4 or CDK6 during the G1 stage in somatic cells. It is a known haploinsufficient tumor suppressor and inhibits the self-renewal of adult stem cells. p18 is detectable as early as the E7 embryo and widely expressed during later mouse embryogenesis. p18 is also broadly present in many adult tissue types, including hematopoietic cells. In contrast, there is virtually little expression of p18 and almost no detectable CDK4-associated activity of p18 protein in mouse ES cells. Correspondingly, loss of p18 results in widespread hyperplasia and organomegaly after birth of the mice. The animals deficient in p18 develop both spontaneous and carcinogen-induced tumors in multiple organs. Moreover, as shown in mice, the correlation of p18 mutation with human glioblastoma further establishes p18 as a tumor suppressor in human. We previously demonstrated that absence of p18 enhances the renewal of HSCs, leading to an increased number of HSCs. However, p18 null T cell leukemia was shown to be transformed in the T cell compartment, not at the level of HSCs. A role of p18 in lung and breast cancer stem cells was also reported. In our current study, genetic manipulations of p18 were performed in a series of embryonic models to define the effect of p18 in ES cell growth as opposed to the previous documented roles of p18 in adult stem cells and tumor cells. 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 and down-regulation of various differentiation markers. Further analysis also revealed that ES cell proliferation was accelerated via up-regulation of CDK4 when p18 was overexpressed. These results demonstrate that p18 stimulates the growth of ES cells, which is opposite to the previously documented roles of p18 in tumor or adult stem cells. Notably, overexpression of p18 was also found to enhance the growth of EB whereas it inhibited the growth of teratoma.