ISL1 and hypopituitarism: Recently, a rare case of identical twin boys with RCCs was reported, suggesting a likely genetic component in RCC development.(15) In a murine progenitor model, conditional deletion of Isl1 (ISL LIM Homeobox 1) resulted in multiple Rathke's cleft-like cysts with 100% penetrance.(16) Pituitary-specific deletion of Isl1 leads to hypopituitarism, which is characterized by increased stem cell apoptosis, reduced differentiation of thyrotropes and gonadotropes, and decreased body size.