The dominant behavioral phenotype of one of the independent loci, Lightweight (Lwt), results from a nonsense 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 in exon 27. Immunoblot analysis indicates an absence of the expected band at 290 kDa in mutant versus wildtype homogenates.