Myotonic dystrophy (DM) is an autosomal dominant trait closely associated with CGT repeat expansions in the same locus on human chromosome 19q13.3. The expansions occur in the 3'untranslated region of a transcription unit encoding a serine-threonine kinase (DM kinase) of a new class based upon structure and function. Lens cataracts are a prominent finding in myotonic dystrophy. DM kinase was shown to be expressed in human and bovine lenses at the RNA level and in human lenses at the protein level. Sequencing of PCR products of RNA extracted from normal human lenses demonstrated an exact match to published genomic and cDNA 3' UTR sequences. Northern blots of bovine lens RNA showed that the transcript is similar in size to the transcript detected in other tissues that are affected in myotonic dystrophy. A polyclonal antibody (DM-2) was produced against recombinant DM protein kinase in rabbits. Development of Western blots with DM-2 showed a single reactive band of 67 kDa. Immunofluorescent studies of formalin-fixed human lens sections detected the DM kinase in the perinuclear cytoplasm of normal human lens epithelial cells and more diffusely in superficial subcapsular cortical fibers. In contrast, the same antibody labeled the nucleus most prominently in a single DM lens.