The study investigates the role of claudin-1 in the epidermal barrier of mammals. By generating claudin-1-deficient mice, the researchers found that these animals died within one day of birth due to severe dehydration and impaired epidermal barrier function. Despite normal keratinocyte layering, the claudin-1-deficient mice exhibited excessive transepidermal water loss (TEWL). Immunofluorescence microscopy and electron microscopy revealed that claudin-1 was absent from tight junctions (TJs) in the epidermis of these mice, while occludin-positive TJs remained present. Tracer experiments showed that TJs in wild-type mice effectively prevented the diffusion of subcutaneously injected tracers, whereas in claudin-1-deficient mice, the tracers passed through the TJs. These findings provide the first evidence that continuous claudin-based TJs are crucial for the barrier function of the mammalian skin. The study also suggests that claudins play a more significant role in the barrier function of stratified epithelia than previously thought.The study investigates the role of claudin-1 in the epidermal barrier of mammals. By generating claudin-1-deficient mice, the researchers found that these animals died within one day of birth due to severe dehydration and impaired epidermal barrier function. Despite normal keratinocyte layering, the claudin-1-deficient mice exhibited excessive transepidermal water loss (TEWL). Immunofluorescence microscopy and electron microscopy revealed that claudin-1 was absent from tight junctions (TJs) in the epidermis of these mice, while occludin-positive TJs remained present. Tracer experiments showed that TJs in wild-type mice effectively prevented the diffusion of subcutaneously injected tracers, whereas in claudin-1-deficient mice, the tracers passed through the TJs. These findings provide the first evidence that continuous claudin-based TJs are crucial for the barrier function of the mammalian skin. The study also suggests that claudins play a more significant role in the barrier function of stratified epithelia than previously thought.