We have recently cloned a novel proteoglycan initially identified in human testis and hence previously called testican. A close examination of the overall protein structure reveals three main regions: four osteonectin/SPARC-like domains encompassing the amino-terminal and central part of the deduced protein, a Kazal-like motif overlapping the third domain, and the CWCV domain in the carboxyl-terminal end region of the protein core. We propose to call it SPOCK, the acronym of SPARC/Osteonectin CWCV and Kazal-like domains proteoglycan, according to its specific multidomain structure. To get further insight into the function, a Northern blot analysis was performed in order to determine the site of expression in various adult tissues; a 5.2 kb transcript appeared only but strongly in mouse brain. The structure of the murine brain proteoglycan was determined through molecular cloning; human and mouse deduced proteins are highly homologous with 95% overall amino acid identity. Murine brain serial sections hybridized with cDNA and immunological probes revealed identical distribution in discrete cerebral regions, such as CA3 hippocampal region and cerebellum. Immunoelectron microscopy showed the antigen selectively localized in the post-synaptic density of scattered pyramidal neurons and Purkinje cells. Structural analysis, a main expression in nervous system and preliminary assignment of the human gene in a critical region for neuropathologies, suggest that SPOCK may be of importance in neural development and neurodegenerative diseases.