This study investigates the role of the p53-p21 pathway in the generation of induced pluripotent stem (iPS) cells. The authors found that deleting the p53 gene or suppressing its function significantly increased the efficiency of iPS cell generation from mouse embryonic fibroblasts (MEF) and human dermal fibroblasts (HDF). Specifically, up to 10% of transduced p53-null MEFs became iPS cells, even without the Myc retrovirus. The p53-null background also allowed the generation of iPS cells from terminally differentiated T lymphocytes. DNA microarray analyses identified 34 p53-regulated genes common in mouse and human fibroblasts, and functional analyses showed that the p53-p21 pathway serves as a safeguard against iPS cell generation. The study concludes that the p53-p21 pathway suppresses iPS cell generation, and transient suppression of p53 may be useful for generating integration-free iPS cells for medical applications.This study investigates the role of the p53-p21 pathway in the generation of induced pluripotent stem (iPS) cells. The authors found that deleting the p53 gene or suppressing its function significantly increased the efficiency of iPS cell generation from mouse embryonic fibroblasts (MEF) and human dermal fibroblasts (HDF). Specifically, up to 10% of transduced p53-null MEFs became iPS cells, even without the Myc retrovirus. The p53-null background also allowed the generation of iPS cells from terminally differentiated T lymphocytes. DNA microarray analyses identified 34 p53-regulated genes common in mouse and human fibroblasts, and functional analyses showed that the p53-p21 pathway serves as a safeguard against iPS cell generation. The study concludes that the p53-p21 pathway suppresses iPS cell generation, and transient suppression of p53 may be useful for generating integration-free iPS cells for medical applications.