Ichthyosis Vulgaris

A 9-year-old boy presents with a 6-year history of dry, rough skin that worsens in winter. His father and two cousins have similar skin findings. On examination, the extensor surfaces of both lower legs below the knees are covered with fine, white-to-grey, adherent scales arranged in a characteristic pattern. The structure marked **A** in the diagram—the polygonal fish-scale scaling on the shins—is a hallmark of ichthyosis vulgaris. This distinctive scaling pattern is caused by loss-of-function mutations in which gene?