RNA interference methodology suppresses gene expression, thus mimicking loss-of-function mutation and enabling in vitro and in vivo gene function analysis. In this study, we used retroviral and lentiviral vectors to deliver small interfering RNAs and report high-efficiency silencing of a green fluorescent protein (GFP) trans gene and the stem cell-specific transcription factors Oct4/POU5F1 and Nanog in human embryonic stem cells. Gene knockdown of Oct4 and Nanog promotes differentiation, thereby demonstrating a role for these factors in human embryonic stem cell self-renewal.