right panel). Additional single nucleotide polymorphisms narrowed the non-recombinant interval to ∼1.5 Mb. Exon sequencing of seven candidate genes within this region revealed a single nonsense point mutation in the mouse homolog of the C. elegans unc-79 gene. The mutation results in a G→T transversion at amino acid position 1292 leading to the conversion of a glutamic acid residue to a premature stop codon (Figure 3B) in exon 27. This change was not found in either the B6 or D2 wild-type strains. Given the reduced body weight and the subsequent discovery (described below) that these mice are hypersensitive to the sedative effects of ethanol, we have assigned the allele name Lightweight to this mutation.
