The mutation rate of ENU depends on many factors, but hovers around 1×10−6 [25]–[27]; therefore, it is unlikely that more than one or two point mutations were induced within this congenic region, and furthermore, the likelihood of mutating a coding exon is very rare [28]. Regardless, sequencing of the majority of coding exons in this congenic interval did not reveal any additional mutations other than Lightweight. Importantly, intercrosses of Lightweight heterozygous animals have confirmed that the homozygous lethality is contained within this congenic interval (D.J.S. and D.C., data not shown). These data are consistent with the previous observation that a homozygous knockout allele of the unc-79 gene is perinatal lethal [29].
