REX-1/Zfp-42 was initially described as a factor present at high levels in murine F9 teratocarcinoma stem cells (EC).The Rex-1 (Zfp-42) gene encodes a zinc finger family transcription factor which is highly expressed in mouse and human embryonic stem cells. It is one of several gene markers used to identify human stem cells. REX-1 is found in spermatocytes and primary oocytes during replication and may play a role in meiosis. Recently, the expression of REX-1 in normal epithelial tissue cells has been demonstrated.