Testicular protein kinase 1 (TESK1) is a serine/threonine kinase highly expressed in testicular germ cells and has the potential to phosphorylate cofilin and induce actin cytoskeletal reorganization. We examined the expression of a lacZ reporter gene linked to a 9.0-kb 5'-flanking region of TESK1 gene in transgenic mice. A high level of lacZ expression was observed in testicular germ cells only at stages after pachytene spermatocytes, the expression patterns being similar to those of TESK1 mRNA in rat testis, determined by in situ hybridization. Expression of lacZ was also detected in renal proximal tubules, cardiac myocytes, and specific neurons in the central nervous system in adult transgenic mice. Whole-mount staining revealed the expression of lacZ in neural tissues in embryonic mice. These results suggest the cell-type- and stage-specific expression of TESK1 gene and the diverse and specific physiological functions of TESK1, including those in spermatogenesis and neural development.