A hygromycin selection cassette replaced a genomic fragment containing sequences encoding the ligand binding and part of the transmembrane domains. RT-PCR analysis on NK fractions of splenocytes derived from homozygous mice demonstrated that no detectable transcript was produced from this allele. Flow cytometry analysis on NK cells, B lymphocytes, macrophages and neutrophils confirmed that no detectable cell surface protein was encoded by this allele. (J:35057)