HDAC1 and infection: We infected OG-MEFs with the OSKM factors in combination with the shRNA viruses corresponding to Hdac1, Hdac2, Hdac3 and Hdac8. On day 8 post-infection, we found that cells infected with viruses containing OSKM and Hdac2 shRNA (shHdac2) had formed approximately 4-fold as many AP-positive colonies as the cells infected with viruses containing OSKM and a control shRNA (Figure 1a).