This mutation was identified in a screen of G3 progeny of an ENU-mutagenized male mouse for developmental and/or functional CD8+ T cell abnormalities. It comprises an A-to-T transversion at nucleotide position 1791 of the gene (Genbank Accession NM_175362.1) that is predicted to result in replacement of leucine by glutamine at amino acid position 525 of the 1159-amino acid protein (L525Q). Western blot analysis detects no CARD11 protein from the mutant allele in the spleen or lymph nodes, and thymic expression is reduced.