Undifferentiated embryonic cell transcription factor 1 (UTF1) is expressed in cells of the inner cell mass and the epiblast. Upon activation by Oct-3/4, UTF1 may be responsible for the rapid proliferation and tumorigenic properties of embryonic stem cells. Sox2 is also part of the Oct-3/4-containing complex able to bind the regulatory region of UTF1. UTF1 acts as a transcriptional coactivator of ATF2 and it binds to the N-terminal region of ATF2. Reduced expression in embryonic stem cells (ESCs) is associated with failure to differentiate properly, although self-renewal is unaffected.