While investigating the effect of tumor suppressor genes on the process of reprogramming to induced pluripotent stem cells (iPSCs) by the three Yamanaka factors (Oct4, Klf4, and Sox2) (Takahashi and Yamanaka, 2006), we tested the three possible combinations of two factors (abbreviated as 2F-OK, 2F-OS, and 2F-KS in reference to Oct4, Klf4, and Sox2) in a series of primary mouse embryo fibroblasts (MEFs) lacking cell cycle regulators and tumor suppressors. This evidence concerns the gene KLF4 and neoplasm.