A 7-year-old girl presents with a solitary, firm, 1 cm, deep dermal nodule on her cheek. The overlying skin has a bluish-red tint, and the lesion feels hard on palpation. Histopathology reveals islands of epithelial cells with anucleate “ghost cells” and calcium deposits. This benign tumor, pilomatricoma, is associated with pathogenic variants in which gene?