Slumber mice exhibit the same phenotype as mayday mice, which contain a mutation in Kcnj8. Although they arose in distinct pedigrees, complementation testing indicated that slumber is allelic with mayday. Sequencing revealed that the slumber allele contains the same mutation as the mayday allele. The mutation in Kcnj8 consists of a deletion of exon 1 and most of exon 2, due to their replacement by an inverted segment of intron 2.