Abstract The phenotypic spectrum of genodermatoses is continuously expanding. Three siblings were referred because of a highly unusual phenotype comprising alopecia, dystrophic nails, palmoplantar keratoderma and trauma-induced skin blistering. Whole-exome sequencing analysis identified a heterozygous large genomic alteration of around 116 0000 bp resulting in the deletion of the KRT9, KRT14, KRT15, KRT16 and KRT19 genes, as well as part of KRT17. This genomic change leads to the generation of a truncated keratin 17 (KRT17) protein encoded by the first three exons of the gene and part of intron 3. The three patients were found to carry the heterozygous genomic deletion while their healthy parents did not, indicative of germline mosaicism. The genomic alteration was found to result in reduced KRT17 expression in patient skin. More importantly, the abnormal truncated KRT17 was found to exert a deleterious effect on keratinocyte cytoskeleton formation, leading to keratin aggregation. Coexpression of wildtype and truncated KRT17 proteins also caused keratin aggregation, demonstrating that the deletion exerts a dominant negative effect. In conclusion, we are reporting on a novel clinical phenotype that was found to result from germline mosaicism for a large genomic deletion spanning six keratin genes, thus expanding the spectrum of clinical manifestations associated with keratin disorders. What is already known about this topic? Various conditions known as keratinopathies have been shown over recent years to be associated with dominant or recessive variants in several individual keratin genes. What does this study add? We report three patients presenting with a unique clinical phenotype that was found to result from germline mosaicism for a large genomic deletion spanning six keratin genes.The genomic variant is predicted to result in a truncated form of keratin 17, which was found in an in vitro assay to disrupt keratinocyte cell cytoskeleton formation.