Functions as a transcriptional activator playing a crucial role during development. Functions in trophoblast differentiation and later in gastrulation, regulating both mesoderm delamination and endoderm specification. Plays a role in brain development being required for the specification and the proliferation of the intermediate progenitor cells and their progeny in the cerebral cortex. Also involved in the differentiation of CD8+ T-cells during immune response regulating the expression of lytic effector genes. Expressed in CD8+ T-cells. Originally expressed in the trophoectoderm of the blastocyst and later in the extraembryonic ectoderm of the early post-implantation embryo. In the embryo proper, expressed in the posterior part of the epiblast. During gastrulation, extends distally into the primitive streak and nascent mesoderm. Also expressed in the developing forebrain and the olfactory lobes. Expressed at 12.5 dpc and 14.5 dpc in the forebrain. Up-regulated in CD8+ T-cells upon activation. Embryonic lethal due to peri-implantation defects. Mutant embryos arrest soon after implantation and fail to form organized embryonic or extraembryonic structures. Conditional mutants, with expression abrogated in the inner cell mass of embryos from early implantation stages onward, display gastrulation defects.