We found that approximately 5% of human pluripotent stem cell lines carry inactivating mutations in the master tumor suppressor TP53 (p53). This work is now published online in the journal Nature. The mutations we observed are frequently seen in cancer, and also lead to increased growth or survival in cultured stem cells. Since human stem cell lines are being therapeutically transplanted into humans, these findings provide an opportunity to identify and discard potentially dangerous cell preparations. This work was initiated in the laboratories of Kevin Eggan and Steve McCarroll at Harvard University and the Broad Institute, and completed after the establishment of the Merkle Lab.