Epidermolytic hyperkeratosis (EH) is a skin disease characterized by keratin filament clumping and degeneration in terminally differentiating epidermal cells. We have discovered that the genetic basis for EH resides in mutations in differentiation-specific keratins. Two of six distinct incidences of EH had a keratin 10 (K10) point mutation in a highly conserved arginine. Remarkably, this same residue is mutated in the basal epidermal K14 in three incidences of another skin disease, epidermolysis bullosa simplex (EBS). By genetic engineering, gene transfection, and 10 nm filament assembly, we show that this mutation is functionally responsible for the keratin filament clumping that occurs in basal (EBS) or suprabasal (EH) cells. These studies strengthen the link between filament perturbations, cell fragility, and degeneration first established with EBS. They also suggest a correlation between filament disorganization and either cytokinesis or nuclear shape, giving rise to the seemingly binucleate cells typical of EH.