Key to the maternal-embryonic dialogue during early implantation are the extra embryonic trophoblast cells that play several critical roles including invasion of the maternal decidua. Cytotrophoblast proliferation and differentiation along an invasive pathway is regulated by a positive gradient of oxygen tension. We have employed transcript profiling to characterize the extra-embryonic tissues during early human implantation. We probed a cDNA library isolated from the ectoplacental cone region of stage 7.5 days post coitus mouse embryos with cDNA from hypoxic cultured human trophoblast cells. This identified the pattern of expression of a series of previously unknown genes that correlate to day 20 of human embryonic development. One EST was selected for further analysis since it is identical to the 1588-bp CGI-15 sequence assembled in silico. The human gene is organized into 10 exons 2063 bp in length. The open reading frame contains 1272 bp predicting protein composed of 365 amino acids. This gene is expressed in many tissues including those found in the reproductive tract. Its expression is regulated by oxygen tension and is unaffected by estradiol or progesterone treatment.