Sorbitol dehydrogenase (Sord) catalyzes the interconversion of sorbitol and fructose and is functionally important both in the metabolism of dietary sorbitol and as a source of fructose in semen. Together with aldose reductase, Sord forms the polyol pathway, which plays an important role in the etiology of diabetic complications. The Sord-deficient mouse (C57BL/LiA) is very useful in animal model studies of the involvement of the polyol pathway in both diabetic and congenital cataracts. To understand more about this strain, we characterized the molecular basis underlying this Sord deficiency and found that this was due to a point mutation in the exon 8/intron 8 junction. Substitution of an A for G at the first position of the strictly conserved GT donor completely abolished normal splicing of exon 8. Aberrant splicing of this junction generates at least three types of transcripts: one lacking exon 8, another that has a truncated exon 8, and a third that contains intron sequences. We have devised two convenient PCR-based methods to identify this mutation in C57BL/LiA mice. These methods are useful in animal experiments that involve cross-breeding with these mice because they allow early determination of genotype without the need to sacrifice the animals for enzyme assay.