The mutation, discovered in an ENU mutagenesis screen for defective natural killer (NK) cell function, is a T-to-A transversion at nucleotide position 31788 of the gene (Genbank genomic region NC_000067) within intron 19, thirteen base pairs upstream from the 5' end of exon 20; it impairs the acceptor splice site of intron 19. cDNA sequence analysis demonstrates the presence of several abnormal transcripts. In one case, a sequence within exon 20 (TCCTAG, position 31850-31855) is used as an acceptor splice signal, resulting in deletion of 55 nucleotides from the 5' end of exon 20; a frameshift occurs after codon 544 that introduces a premature stop codon following codon 561. Some normal transcripts may also be produced.