We report the cloning and expression pattern of a novel Ste20-type kinase gene, NIK-related kinase (Nrk), located on the mouse X chromosome. The full-length Nrk cDNA encodes a 1455-amino-acid polypeptide characterized by a N-terminal Ste20-type catalytic domain and a C-terminal regulatory domain characteristic of the group I GCK subfamily. The overall structure of the NRK protein is closely related to that of Nck-interacting kinase (Nik). In situ hybridization revealed that Nrk was predominantly expressed in skeletal muscle during mouse embryogenesis. Nrk gene expression was detected in the myotome at 10.5 dpc and, thereafter, was observed in developing skeletal musculature from 11.5 to 13.5 dpc. However, expression in skeletal muscle was not observed in adults.