ATP6AP1 and cancer: In this study, we expand the spectrum of rare cancer types underpinned by likely pathognomonic genetic alterations, as loss-of-function mutations targeting ATP6AP1 or ATP6AP2 are present in up to 72% of GCTs, but are found in less than 0.1% of common cancer types (Fig. 3a, b) and in none of the histologic mimics of GCTs tested here (Fig. 3c).