Levi Garraway and colleagues report the identification of somatic mutations of RNF43, which encodes an E3 ubiquitin ligase that negatively regulates Wnt signaling, in over 18% of colorectal adenocarcinomas and endometrial carcinomas. We report somatic mutations of RNF43 in over 18% of colorectal adenocarcinomas and endometrial carcinomas. RNF43 encodes an E3 ubiquitin ligase that negatively regulates Wnt signaling. Truncating mutations of RNF43 are more prevalent in microsatellite-unstable tumors and show mutual exclusivity with inactivating APC mutations in colorectal adenocarcinomas. These results indicate that RNF43 is one of the most commonly mutated genes in colorectal and endometrial cancers.