Junctional epidermolysis bullosa (JEB), a genetically heterogeneous group of blistering skin diseases, can be caused by mutations in the genes encoding laminin 5 or collagen XVII, which are components of the hemidesmosome-anchoring filament complex in the skin. Here, a family with severe nonlethal JEB and with mutations in genes for both proteins was identified. The index patient was compound heterozygous for the COL17A1 mutations L855X and R1226X and was heterozygous for the LAMB3 mutation R635X. As a consequence, two functionally related proteins were affected. Absence of collagen XVII and attenuated laminin 5 expression resulted in rudimentary hemidesmosome structure and separation of the epidermis from the basement membrane, with severe skin blistering as the clinical manifestation. In contrast, single heterozygotes carrying either (1) one or the other of the COL17A1 null alleles or (2) a double heterozygote for a COL17A1 and a LAMB3 null allele did not have a pathological skin phenotype. These observations indicate that the known allelic heterogeneity in JEB is further complicated by interactions between unlinked mutations. They also demonstrate that identification of one mutation in one gene is not sufficient for determination of the genetic basis of JEB in a given family.