The mutant phenotype, discovered in an ENU mutagenesis screen for impaired response of peritoneal macrophages to Toll-like receptor (TLR) ligands, is due to a T-to-C transition at nucleotide position 1284 (Genbank Accession NM_031178) in the second of two exons. It results in replacement of leucine by proline at amino acid position 393 (L393P), the initial leucine in the fourteenth extracellular leucine-rich repeat of the receptor's ectodomain.