Retrovirus was made by transfecting 293FT cells with pUMVC and pCMV-VSV-g plasmids, and individual plasmids: pMXs-hKLF4, pMXs-hSOX2, pMXs-hOCT4 or pMXs-hcMYC (Addgene, Cambridge, MA, USA) at a 1:1:1 ratio using Lipofectamine 2000 (Invitrogen, 11668-019). Virus supernatant was collected and combined, filtered through a 0.45-μm filter (EMD Millipore, SCHVU01RE, Billerica, MA, USA) and concentrated 80–100-fold in a centrifugal filter with a 1 00 000 MWCO (EMD Millipore, UFC910008). The concentrate containing all four factors was then used immediately to transduce fibroblasts at a final concentration between 3- and 12-fold.
