ENU induced a T to C transition at position 12038 (GenBank NC_000068). The mutation affects the donor splice site of intron 2. cDNA sequencing of the four largest Eif2ak4 transcripts in atchoum mice revealed that the mutation invariably results in skipping of exon 2, in some cases along with skipping of other exons; splicing from exon 1 to exon 3, exon 4, exon 5, or exon 6 was observed in the four sequenced transcripts. No GCN2 protein expression was detected in homozygous atchoum macrophages by immunoblot.