Gene expression during oocyte maturation, fertilization, and early embryo development until zygotic gene activation is regulated mainly by translational activation of maternally derived mRNAs. This process requires the presence of a poly(A)-binding protein. However, the cytoplasmic somatic cell poly(A)-binding protein (PABP1) is not expressed until later in embryogenesis. We recently identified an embryonic poly(A)-binding protein (ePAB) in Xenopus. ePAB is the predominant cytoplasmic PABP in Xenopus oocytes and early embryos and prevents deadenylation of mRNAs, suggesting its importance in the regulation of gene expression during early Xenopus development. Here we report the identification of the mouse ortholog of Xenopus ePAB. The mouse ePAB gene on chromosome 2 contains 14 exons that specify an alternatively spliced mRNA encoding a protein of 608 or 561 aa with Ϸ65% identity to Xenopus ePAB. Mouse ePAB mRNA is expressed in ovaries and testis but not in somatic tissues. In situ hybridization localizes ePAB RNA to oocytes and confirms its absence from surrounding somatic cells in the mouse ovary. During early development, mouse ePAB is expressed in prophase I and metaphase II oocytes and one-cell and two-cell embryos and then becomes undetectable in four-or-more-cell embryos. In contrast, PABP1 mRNA expression is minimal in oocytes and early embryos until the eight-cell stage when it increases, becoming predominant at the blastocyst stage. The expression of mouse ePAB before zygotic gene activation argues for its importance in translational activation of maternally derived mRNAs during mammalian oocyte and early preimplantation embryo development.translational activation ͉ embryogenesis ͉ oogenesis M echanisms for establishing the germline and carrying out oogenesis in evolutionarily distant animals exhibit common themes. Gametes develop from primordial germ cells that are set aside during early embryogenesis (1). In most metazoans, primordial germ cells have an extragonadal origin and migrate to reach the somatic gonad, where they proliferate by mitosis to form oocytes (1). Oocytes, in turn, enter meiosis only to be arrested at the prophase of the first meiotic division (2, 3). This first meiotic arrest may last up to a few years in Xenopus and several decades in humans and is characterized by synthesis and storage of large quantities of dormant mRNA (4, 5). The resumption of meiosis is stimulated by progesterone in Xenopus (6, 7) or by gonadotropins in mouse and human (8, 9) and marks the onset of oocyte maturation.Oocyte maturation is accompanied by a complex network of translational activation and repression of dormant maternal mRNAs (10-13), whereas transcription is limited at best. These maternal mRNAs drive the oocyte's reentry into meiosis and control the rate of mitosis during the early cleavage divisions of the embryo (13-16). The transcriptional silencing that begins with oocyte maturation persists during the initial mitotic divisions of the embryonic cells. In Xenopus, after 12 rapid synchronous cle...