Dominant-negative mutations in the KRT1 and KRT10 genes cause epidermolytic hyperkeratosis, a rare form of ichthyosis sometimes associated with palmoplantar keratoderma. Although there is no permanent cure, some patients improve on retinoid therapy. More knowledge is needed, however, about the mechanism of action of retinoids and the genotypic/phenotypic correlations in this disease. Thirteen patients from 10 families with generalized disease and 2 sporadic patients with nevoid lesions were studied, probably representing most of the patients in Sweden and Norway. All patients, except one nevoid case, were known to have KRT1 or KRT10 mutations. Those with mutated keratin 1 (K1) invariably had associated keratoderma (n=6). In contrast, only 1 of 7 patients with K10 mutations had this problem (p = 0.0047). Five out of 6 patients with KRT10 mutations benefited from treatment with oral acitretin (5-25mg/day) or topical tretinoin/tazarotene, but none of the patients with KRT1 mutations derived any benefit. Quantitative analysis of K1 and K10 mRNA in skin biopsies obtained before and after retinoid therapy (n=8) showed no consistent down-regulation of mutated keratin that would explain the therapeutic outcome. Instead, the mRNA expression of K2e (a normal constituent of the upper epidermis) diminished especially in nonresponders. In contrast, K4 mRNA and protein (marker of retinoid bioactivity in normal epidermis) increased in almost all retinoid-treated patients. In conclusion, our study confirms a strong association between KRT1 mutations and palmoplantar keratoderma. Retinoid therapy is particularly effective in patients with KRT10 mutations possibly because they are less vulnerable to a down-regulation of K2e, potentially functioning as a substitute for the mutated protein in patients with KRT1 mutations.