Overexpression of Klf4 results in sustained Oct-4 expression. (A) Immunoprecipitation studies demonstrating that the exogenously introduced (FLAG epitope-tagged) Klf4 and SOCS-3 proteins are expressed in transduced ES cells and that, consistently, total Klf4 and SOCS-3 protein levels are increased compared to cells transduced with vector alone. (B) Evaluation of Oct-4 protein levels in d6 EBs derived from pMIEG3-, pMIEG3-SOCS-3–, and pMIEG3-Klf4–transduced ES cells. Oct-4 band intensities normalized to β-actin band intensities are shown graphically. IP indicates immunoprecipitation; IB, immunoblotting.