Epidermolysis bullosa simplex (EBS) arises from mutations within the keratin 5 and 14 (K5 and K14) genes which alter the integrity of basal keratinocytes cytoskeleton. The majority of these defects are missense mutations in the rod domain, whose locations influence the disease severity. We investigated a large family dominantly affected with the Dowling-Meara form of EBS (EBS-DM). Sequencing of amplified and cloned K5 cDNA from cultured keratinocytes revealed a 66 nucleotide deletion in one allele corresponding to the last 22 amino acid residues encoded by exon 1 (Val164 to Lys185). Sequencing of amplified genomic DNA spanning the mutant region revealed a heterozygous G-to-A transition at +1 position of the consensus GT donor splice site of intron 1 of K5. This mutation leads to the use of an exonic GT cryptic donor splice site, located 66 nucleotides upstream from the normal donor splice site of intron 1. The corresponding peptide deletion includes the last five amino acids of the H1 head domain and the first 17 amino acids of the conserved amino terminal end of the 1A rod domain, including the first two heptad repeats and the helix initiation peptide. The shortened polypeptide is expressed in cultured keratinocytes at levels which are comparable to the normal K5 protein. This is the first splice site mutation to be reported as a cause of EBS-DM. Owing to the functional importance of the removed region, our data strongly suggest that shortened keratin polypeptide can impair keratin filament assembly in a dominant manner and causes EBS-DM.