Epidermolytic hyperkeratosis is an autosomal dominant ichthyosis characterized by blistering, especially at birth and during childhood, and hyperkeratosis. Epidermolytic hyperkeratosis presents striking clinical heterogeneity, particularly between families. Several avenues of research have implicated an abnormality of epidermal differentiation in the pathogenesis of this disease. In a three-generation family with 20 affected individuals, we tested a variety of candidate loci and identified linkage to the type II keratin region on chromosome 12. Further investigation revealed a mutation in the H1 subdomain of the keratin 1 gene as the cause of EHK in this family. Because keratin 10 is the co-expressed partner of keratin 1, it was not surprising when abnormalities in keratin 10 were found in other families with EHK. We have examined 52 patients from 21 families and have identified at least six clinical phenotypes. The most useful distinguishing feature was the presence or absence of severe hyperkeratosis of the palms and soles. We and others are continuing to search for and characterize mutations in keratin 1 and 10 in patients with epidermolytic hyperkeratosis. Correlation of the clinical disease types with the specific mutations should lead to a better understanding of the relationship between keratin structure and function in normal and diseased epidermis.