Zep, a novel 49 kDa zinc finger protein, was found in the brain of day-13 mouse embryos and cloned. Zep contains two C2H2-type zinc finger motifs close to the N-terminal region. The majority of the molecule is composed of a proline-rich domain showing similarity to proline-rich domains in transcription factors and a salivary proline-rich protein. In addition to the proline-rich domain, Zep has an acidic domain and a serine/threonine-rich domain, all of which are frequently found in many transcription factors. The overall organization of Zep shows no similarity to any other proteins. There is a nuclear localization signal in Zep, and the Zep-GFP (green fluorescent protein) fusion protein is located predominantly in the nucleus. In the day-13 mouse embryo, Zep is strongly expressed in the nervous system, i.e. brain, spinal cord, and dorsal root ganglia, with strong to weak expression observed in other regions. Zep continues to be strongly expressed in the neonatal brain; however, its expression is weak in the brain and spinal cord of adult mice. In situ hybridization reveals strong signals for Zep mRNA in the cerebellum and olfactory bulb with moderate signals detected in the hippocampus and cortex. Strong Zep expression is observed in adult thymus, lung, spleen, testis, and ovary. Zep may be involved in the formation and remodeling of various tissues including nervous tissue, probably through transcriptional regulation.