pLentG-KOSM is a simple lentiviral vector in which defined factors are in-frame fused into a single open reading frame (ORF) via self-cleaving 2A peptides and are controlled by a CMV promoter. The mouse transcription factor ORF is followed by IRES-GFP as a reporter for viral transduction. This polycistronic expression system can efficiently reprogram mouse or human somatic cells into iPS cells; for example, in infected MEF (GFP+), the reprogramming efficiency is around 1%, which is at least 10-fold higher than the reprogramming method using a pool of viruses in which each virus expresses a different stem cell factor gene. Notably, most of the iPS cells generated using the pLentG-KOSM system contain only a single copy of the viral vector (Ref. 1).