Epidermolytic hyperkeratosis (EH) is a rare autosomal dominant skin disease. Recent studies in our laboratory established genetic linkage to the type II keratin gene locus on chromosome 12q in one family with EH and identified a single amino acid mutation in keratin 1 that is responsible for the disease. Other point mutations in the keratin 1 or keratin 10 genes have now been reported in other patients with EH. We have examined a series of probands with EH in order to develop a catalog of mutations in keratin 10. Using direct sequencing of PCR-amplified genomic DNA, we have identified mutations in six families, in which five mutations occur in the beginning of the 1A rod domain of keratin 10-namely, two ARg10 to His, one Arg10 to Cys, and Asn8 to His, and a Tyr14 to Asp. This region contains highly conserved residues among all keratins. An additional mutation (Leu103 to Gln) was found in the conserved region late in the 2B rod domain in keratin 10. We developed several allele-specific assays to assess the frequency of these mutations in the general population. No evidence was found for the presence of such changes in unaffected individuals. In vitro functional assays performed with peptides corresponding to the 1A mutations in these families show severely diminished capacity to disaggregate preformed keratin intermediate filaments, in comparison with a wild-type control peptide. Results from this work support the hypothesis that the beginning of the 1A rod domain segment in keratin 10 contains preferential sites for disease-causing mutation in EH. This should be of considerable use when developing prenatal diagnostic tests and biologically based therapies for this disease.