A neomycin resistance cassette replaced a small DNA segment of the coding region (+69 to 94). A 300 bp segment in the 5' flanking region containing CT repeats was also deleted. Northern blot analysis on thymus and spleen RNA from homozygous mice demonstrated that no detecable transcript was produced from this allele. Flow cytometry experiments on leukocytes derived from homozygous mice confirmed that no detectable cell surface protein was expressed from this allele. (J:75412)