Nanog (Homeobox protein NANOG) is a transcription regulator involved in inner cell mass and embryonic stem (ES) cells proliferation and self-renewal. Nanog imposes pluripotency on ES cells and prevents their differentiation towards extraembryonic endoderm and trophectoderm lineages. Nanog blocks bone morphogenetic protein-induced mesoderm differentiation of ES cells by physically interacting with SMAD1 and interfering with the recruitment of coactivators to the active SMAD transcriptional complexes.