Csn8 is the smallest subunit of the constitutively photomorphogenic 9 (COP9) signalosome (CSN), which consists of eight distinct components. CSN is homologous to the lid subcomplex of the 26S proteasome and is a regulator of the ubiquitin-proteasome pathway. Murine Csn8 is an ortholog of Arabidopsis COP9, which was originally identified as a key regulator of photomorphogenic development. CSN is essential for the viability of plants and flies, but the role of this conserved protein complex in development of vertebrate animals remains obscure. We report the genomic structure of murine Csn8 gene and the expression pattern of Csn8 during early embryo development. Mouse Csn8 protein contains 209 amino acid residues and is encoded by a single gene located on chromosome 1. Csn8 is expressed in embryonic stem (ES) cells and throughout early embryo development from zygote, preimplantation embryos, to post-implantation embryos. Immunostaining studies revealed that Csn8 is predominantly present in the inner cell mass (ICM) of E3.5 blastocyst and is widely expressed in E9.5-day embryos.