A T to A transversion at position 596 of the transcript created a premature stop codon in place of cysteine 199. Northern blot revealed abundant transcript in mutant DP thymocytes, putatively encoding an N-terminally truncated protein lacking the targeting, regulating, and catalytic domains. Western blot detected no full-length protein. Kinase activity was about 50% less than that in wild-type thymocytes.