The cell types expressing cot proto-oncogene mRNA were identified by in situ hybridization (ISH) histochemistry. Among a variety of adult mouse tissues examined, four types of glandular cells expressing cot gene were identified: (1) granular duct cells in the submandibular and sublingual glands; (2) serous cells in the parotid gland; (3) peptic (chief) cells in gastric glands; and (4) goblet cells in colonic glands. Investigation of the developmentally regulated expression of cot mRNA using tissues of 14-day and 18-day embryos, newborn and weanling mice showed that cot gene is expressed only in morphologically differentiated and functionally activated cells of these four types. No other types of cells showing ISH signals were observed. Based on these results, cot gene expressions in cultured cells of colonic adenocarcinomas and gastric adenocarcinomas were examined. SW 480 and WiDr cells showed high expression of this gene and so should be useful for functional analysis of Cot kinase. The expression patterns of cot gene in tumor tissues of the parotid gland, and gastric and colonic glands were investigated. Two of the tissues over-expressed this gene markedly, suggesting that overproduction of Cot kinase may be one cause of their transformation.