This mutation was discovered among G3 progeny of an ENU mutagenized mouse in a screen for susceptibility to induction of colitis by low doses of dextran sodium sulfate (DSS). The molecular change, in the second, last exon of the gene, results in a G to T transversion at nucleotide position 1944 of the mRNA that replaces the neutral arginine at amino acid position 648 with the polar leucine (R643L).