Inbred mouse strains are widely used for genetic studies because of the isogenicity within a strain or F1 hybrid and the genetic heterogeneity between inbred strains. In the process of modifying Disc1 in the mouse genome, a 25-bp deletion was discovered in exon 6 of the gene in the 129S6/SvEvTac inbred strain, and subsequently in 16 other inbred strains in the category known as ‘Castle’s mice’. The deletion (Disc1del ) induces a frame shift that introduces a premature termination codon, which has been shown to confer an impairment in working memory. To extend knowledge of the distribution of Disc1del among the various inbred strains of laboratory mouse, we investigated whether Disc1del is present in the categories known as ‘Swiss mice’ and ‘strains derived from China and Japan’. We found that the FVB/NJ, SJL/J and SWR/J strains in the ‘Swiss mice’ category and DDY/JclSidSeyFrkJ in the ‘China and Japan’ category are homozygous for the Disc1del allele, while ICR/HaJ in the ‘Swiss mice’ category is homozygous for wild-type Disc1. Since the Disc1del -positive strains FVB and SJL are commonly used for the generation of transgenic mice, and thus contribute to the genetic background of multiple transgenic lines, our results may allow scientists to avoid the potential confounding effects of the Disc1del allele in transgenic studies of learning and memory.