During the course of reviewing the results of automatic annotation of 1989 clusters derived from 16,078 sequences of an adult mouse testis cDNA library, we found a cluster of 24 cDNA clones showing similarity to the mouse neuronal protein 15.6 (NP15.6) gene andnamedit the NP15.6-like gene. This NP15.6-like gene correspondedto an intronless gene, mappedto chromosome 15, andwas expressedpredominantly in the testes. Interestingly, the mouse NP15.6 gene itself is a split gene consisting of three exons, mappedto chromosome X, andwas expressedat high levels in various tissues andorgans. We foundtwo more intronless NP15.6-like genes; one was mappedto chromosome 5, andthe other to chromosome X. These two intronless genes were probably processed-type pseudogenes. Our present observations support the idea that NP15.6 family proteins have a shorter half-life andspan a membrane.