Mouse embryonic stem (ES) cell self-renewal depends upon extrinsic signals from leukemia inhibitory factor (LIF) and bone morphogenetic protein (BMP). These molecules activate, respectively, the nuclear localization of the latent transcription factor STAT3 and the expression of Id genes. In contrast, the homeodomain proteins Oct4 and the recently identified Nanog are intrinsic factors required for maintenance of the undifferentiated state. When overexpressed, Nanog allows ES cells to self-renew in the absence of the otherwise obligatory LIF and BMP signals. However, the highest efficiency of ES cell self-renewal occurs when Nanog is overexpressed and cells are exposed to LIF. In contrast, when Oct4 is overexpressed, ES cells differentiate in a similar manner to the differentiation that occurs upon LIF withdrawal. These observations are brought together to provide a genetic model of ES cell self-renewal centered upon interactions between Oct4, STAT3 and Nanog.