The endogenous gene was disrupted by the insertion of a cassette containing lacZ and PGK-neo. Beta galactosidase activity was detected in mutant mice indicating the production of a fusion protein predicted to consist of the extracellular, transmembrane, and juxtamembrane domains followed by beta-galactosidase. The protein putatively lacks the entire tyrosine kinase and C-terminal domains, but the kinase-independent functions are expetcted to remain intact. (J:90078)