PIK3R1 and endometrial cancer: Consistent with this prediction, a fifth of the nonsense mutations observed in the 369 cancer genes occurred in just 8 tumor suppressor genes (APC, ATM, PTEN, MGA, PIK3R1, ARID1A, NF1, and FAT1)—all target genes expected for the tumor types where POLE mutations arise, namely colorectal and endometrial cancers.