Preimplantation development in the mouse is characterized by the occurrence of several critical genetic and epigenetic events. Until recently, very little was known about the regulation of these events. The search for genes which are involved in the control of the earliest stages of mouse development has so far resulted in only a few candidates. Oct-4, a member of the POU transcription factor family, is encoded by a gene belonging to this group. Initially present as a maternal factor in the oocyte, Oct-4 is expressed by the embryo throughout the preimplantation period, as well as in germ cell precursors of adult mice. Oct-4 expression is correlated with an undifferentiated phenotype, both in the embryo and in cell lines derived from it. Regulation of the Oct-4 gene is dependent on the activity of two separate enhancers, one of which is specifically active in pluri- and totipotent cells. Its function as a transcriptional regulator is supported by the identification of an increasing number of potential target genes, including some known to be essential for early embryonic development.