RBPJ and Hyperkeratosis: The hyperplastic epidermis, acanthosis, hyperkeratosis, parakeratosis, and mast cell infiltration were evident in RBP-jCKO skin as early as 1 wk after birth, followed by dramatic dermal mast cell accumulation, serum IgE elevation, and systemic Th2 cell expansion in adult RBP-j–deficient animals (Figure 1A–1C).