The gene was truncated by replacement of exon 8 sequences encoding the distal half of the cytoplasmic domain with a neomycin resistance cassette. A tyrosine to phenylalanine substitution at residue 343 (Y343F) was also introduced. Expression of the mutant allele was confirmed by RT-PCR analysis of bone marrow RNA from homozygous mutant animals. Sequence analysis of RT-PCR products verified the presence of the Y343F amino acid substitution. (J:70036)