Mesothelin is a glycosylphosphatidylinositol-linked glycoprotein highly expressed in mesothelial cells, mesotheliomas, and ovarian cancer, but the biological function(s) of the protein is not known. We have analyzed the expression of the mouse mesothelin gene in different developmental stages and in various adult tissues by Northern hybridization. The 2.5-kb mesothelin transcript was detected in the mRNA of E 7.0, E 15.0, and E 17.0 stages of mouse development. In adult tissues the mesothelin gene was expressed in lung, heart, spleen, liver, kidney, and testis. To directly assess the function of the mesothelin in vivo, we generated mutant mice in which the mesothelin gene was inactivated by replacing it with the neomycin resistance gene. In homozygous mutant mice neither mesothelin mRNA nor the protein product was detected. Null mutant mice were obtained in accordance with Mendelian laws, and both males and females produced offspring normally. No anatomical or histological abnormalities were detected in any tissues where mesothelin was reportedly expressed in wild-type mice. Our results demonstrate that mesothelin function is not essential for growth or reproduction in mice.