Does BCAR4 have a role in mammalian embryo development? Expression, localization and functional data support that BCAR4 is a maternal-effect protein in non-rodent mammals. BCAR4 was previously identified as an oocyte-specific gene in cattle, and as a marker of certain breast tumors in humans. Human oocytes were obtained from patients undergoing IVF, but had failed to mature after ovarian stimulation. Dog oocytes were obtained from ovariectomized bitches. Pig, horse and bovine ovaries were obtained from commercial slaughterhouses for extraction of immature oocytecumulus complexes. In vivo matured bovine matured oocytes were obtained after ovulation induction and ovulation inducing treatment of Montbeliard heifers. Expression at the RNA level was analyzed by reverse transcription coupled to polymerase chain reaction. Western blot and immunolabeling coupled to confocal or electronic microscopy were used to analyze bovine protein expression and intracellular localization. For the functional approach, short-interfering RNA were microinjected into mature bovine oocytes, followed by IVF; cleavage and embryo development were recorded. The BCAR4 gene is conserved in mammalian species from various orders and has been lost in rodents after divergence with lagomorphs. The transcript is expressed in the oocytes of humans and domestic species. We bring the first experimental evidence of the BCAR4 protein in mammals. In cattle, the protein is not detected in immature oocytes but starts to be synthesized during maturation, increases in the zygote and persists until the morula stage. The protein is detected throughout the cytoplasm in mature oocytes, concentrates in and around the pronuclei in the zygote, and appears to shuttle in and out of the nuclei starting in the 2-cell embryo; BCAR4 is also present at the junctions between blastomeres from 2-cell to morula. In our functional approach, targeting the BCAR4 transcript by small-interfering RNA significantly compromised development to the morula or/and blastocyst stages (P 0.05, logistic regression). As indicated above, protein expression and function were investigated in cattle and mostly in vitro matured oocytes were used. This study provides a novel candidate gene whose mutation or deregulation may underlie certain cases of unexplained female infertility. This work was sponsored by grants from the French Ministry of Research (03P409), Agence Nationale de la Recherche (ANR-07-GANI-004-01) and Apisgene. No competing interests declared.