The expression of the mouse IAP promoted placental (MIPP) gene was examined in the tissues of the 13.5 day mouse conceptus by Northern hybridization analysis. MIPP transcripts were found in total cellular RNA isolated from the placenta, parietal yolk sac, and visceral yolk sac, but not the embryo or amnion. Expression in the visceral yolk sac was confined to the visceral endoderm cell layer suggesting that the MIPP gene is specifically expressed in tissues derived from the trophectoderm and primitive endoderm cell lineages. The expression of IAP genes paralleled that of MIPP in all tissues studied except for the presence of IAP transcripts in the amnion.