Pluripotent stem cells (PSCs) hold significant promise in regenerative medicine 
due to their unlimited capacity for self-renewal and potential to differentiate 
into every cell type in the body. One major barrier to the use of PSCs is their 
potential risk for tumor initiation following differentiation and 
transplantation in vivo. In the current study we sought to evaluate the role of 
the tumor suppressor Pten in murine PSC neoplastic progression. Using eight 
functional assays that have previously been used to indicate PSC adaptation or 
transformation, Pten null embryonic stem cells (ESCs) failed to rate as 
significant in five of them. Instead, our data demonstrate that the loss of Pten 
causes the emergence of a small number of aggressive, teratoma-initiating 
embryonic carcinoma cells (ECCs) during differentiation in vitro, while the 
remaining 90-95% of differentiated cells are non-tumorigenic. Furthermore, our 
data show that the mechanism by which Pten null ECCs emerge in vitro and cause 
tumors in vivo is through increased survival and self-renewal, due to failed 
repression of the transcription factor Nanog.