The mechanisms responsible for prevention of corpus luteum regression during early pregnancy are diverse and appear to have arisen in concert with the evolutionary divergence of placental structure. That used by the sub-order Ruminantia is unique and involves the production of a Type I interferon (IFN), IFN-tau (tau). Although IFN-tau resembles other Type I IFNs (such as IFN-alpha, -beta, and -omega) in structure as well as in many of its biological properties, it is not virally inducible and is instead produced constitutively by embryonic trophectoderm during the period immediately prior to implantation. The transcription factor Ets-2 is a component of the regulatory mechanism involved in transcription of IFN-tau. These genes probably arose as the result of a duplication of an IFN-omega gene, 36 million years ago, at about the time the Ruminantia sub-order emerged. They have duplicated extensively since then and there may be 10 or more genes in some present-day species. The expression of different IFN-tau is unequal and they differ in biological potency. The rapid evolution of IFN-tau genes possibly reflects the placenta as a site of considerable genetic experimentation.