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