The pathogenic mechanisms underlying myotonic dystrophy (DM), which results from a (CTG) n repeat expansion mutation in the 3'-untranslated region (3'-UTR) of the myotonic dystrophy protein kinase gene ( DMPK ), remain obscure. The multisystemic nature and variable expressivity of the symptoms are unlikely to be explained by a defect in this gene alone. However, the location of the DM-associated (CTG) n repeat in the promoter region of SIX5, immediately downstream of DMPK, implicates it as a second candidate with a pathological role in DM. We hypothesize that dysfunction of SIX5, which is homologous to the Drosophila eye development gene sine oculis ( so ), is primarily responsible for the ophthalmic features of DM. We report an expression pattern for SIX5 in the normal adult eye that matches the sites of the ocular pathology in DM. SIX5 transcripts were detected in the adult corneal epithelium and endothelium, lens epithelium, ciliary body epithelia, cellular layers of the retina and the sclera. SIX5 expression was not detected in fetal eyes. We also report a restricted but partially overlapping expression pattern for DMPK transcripts and DMPK protein in normal fetal and adult eyes. DMPK transcripts were detected in fetal eyes and in adult conjunctival and corneal epithelia, uvea, cellular layers of the retina, optic nerve and in the sclera. DMPK protein was detected in the adult retina, conjunctival and ciliary body epithelia and in the smooth muscle of the ciliary body, pupillary sphincter and uveal blood vessels. We propose that the expression patterns of these two genes indicate their relative contribution to the ophthalmological dysfunction seen in DM. Furthermore, the expression of SIX5 and not DMPK in the adult lens implicates a role for SIX5 dysfunction in the development of adult onset cataracts, the most frequently occurring eye phenotype in DM.