G90 is a novel mouse gene that does not belong to any known gene family. It has previously been shown that this gene is expressed exclusively in post-mitotic cells of the adult mouse intestine and testis, therefore suggesting a role in the control of proliferation and/or differentiation. Here we report the detailed spatio-temporal expression pattern of G90 during mouse embryonic development. We found G90 expression in specific structures of the developing head, namely the brain, inner and middle ear, olfactory epithelium, vomeronasal organ, nasopharynx, oropharynx, papillae of the tongue and oral cavity, pituitary gland and epiglottis. Interestingly, there was a clear correlation between G90 expression and absence of proliferation in most of the cells showing expression of this gene during embryonic development; this finding supported our functional hypothesis.