One approach to studying the function of driver events is through analysis of cancers with unique, extreme, or pathognomonic genotypes (e.g., ultramutation in the context of POLE deficiency in endometrial cancer and loss-of-function mutations in Accessory Protein 1 (ATP6AP1) or ATPase H+ Transporting Accessory Protein 2 (ATP6AP2) in granular cell tumors) that are likely to evoke distinctive molecular and physiological phenotypes (2). The gene discussed is ATP6AP2; the disease is granular cell tumor.