The torpid mutation, identified in an ENU mutagenesis screen for mutants with altered responses to Toll-like receptor (TLR) ligands, corresponds to an A-to-T transversion in the acceptor splice site of intron 6 (AG -> TG) (position 11943 in Genbank genomic region NC_000075). The gene has 7 exons, of which only the last three are translated. The presence of an in-frame stop codon in intron 6 causes premature termination of the protein after the anomalous addition of 3 amino acids, encoded by 9 intronic nucleotides, in place of the six C-terminal amino acids encoded by exon 7.