Epidermolytic hyperkeratosis (EHK), (bullous congenital ichthyosiform erythroderma), is an autosomal dominant human skin disorder. Recently, we and others have described mutations in keratins 1 and 10 (K1 and K10) in patients with this disease. Structure-function models predict that these mutations would impair normal filament assembly and function. We have extended our earlier studies to include 8 more incidences of EHK. In half of these families, we were unable to locate a mutation within the rod domains of either K1 or K10. However, polymorphic restriction site and sequence analysis of the other families revealed a mutational hot spot within the 1A alpha-helical segment of K10. These involve Arginine to Histidine, Arginine to Cysteine and Arginine to Leucine substitutions at residue 10 of the rod domain. Interestingly, mutations in the corresponding Arginine residue in keratin K14 have been identified in patients with epidermolysis bullosa simplex. The large number of mutations found at this position in both keratins K10 and K14 suggests that other epithelia cell disorders will be discovered that are caused by the corresponding mutation in related type I keratin genes.