For the generation of human iPS cells, primary HFF were infected with an equal ratio of retroviruses (Oct4, SOX2, KLF4 and c-MYC) by spinfection of the cells at 1850, r.p.m. for 1 h at room temperature in the presence of polybrene (4 μg ml−1). After two serial infections, cells were passaged onto fresh mouse embryonic fibroblasts and switched to hES medium containing DMEM/F12 (Invitrogen) supplemented with 20% Knockout Serum Replacement (Invitrogen), 1 mM L-glutamine, 0.1 mM non-essential amino acids, 55 M -mercaptoethanol and 10 ng ml−1 bFGF (preprotech). For the derivation of hiPS cells lines, iPS-like colonies were manually picked and maintained on fresh mouse embryonic fibroblast feeder layers for five passages before being transferred onto Matrigel/mTesR1 conditions (n=2 WTiPSC clones). iPSCs with a knockdown of p53 were generated as described49. Briefly, HFF were infected with lentiviral particles expressing a shRNA against p53 before iPSC reprogramming (n=3 p53KDiPSC clones).
