Germ cell nuclear factor (GCNF, NR6A1) is an orphan member of the nuclear receptor superfamily and functions as a repressor of gene transcription. GCNF mRNA is expressed in postgastrulation mouse embryos and is required for normal mouse embryonic development. In adult mice, GCNF transcripts are predominantly expressed in spermatogenic cells and growing oocytes of the gonads. To extend this observation to the protein level, we generated and characterized a specific antibody against GCNF. Using this antibody we found that GCNF protein was exclusively present in postmeiotic spermatogenic cells of the testis in 21- and 56-day-old mice. In the ovary, GCNF protein was present in the cytoplasm of oocytes from primary to preovulatory follicles. GCNF protein was also present in unfertilized oocytes and preimplantation embryos. The presence of GCNF protein in adult mouse gonads indicates that GCNF may play a role during gametogenesis. Our results also show that GCNF in early embryos is a maternal protein and could be involved in the regulation of zygotic gene expression and preimplantation embryonic development.