CTNNB1 encodes the protein b-catenin, a transcriptional activator involved in the WNT signaling pathway. Somatic gain-of-function mutations in CTNNB1 result in aberrant accumulation of the b-catenin protein and are prevalent in a wide range of solid tumors, including endometrial carcinoma, ovarian carcinoma, hepatocellular carcinoma, and colorectal carcinoma, among others. Cancers with CTNNB1 mutations are presumed to be resistant to pharmacologic inhibition of upstream components of the WNT pathway, instead requiring direct inhibition of b-catenin function. Genetic alterations in CTNNB1 have been identified in 4% of non-small cell lung cancers. The CTNNB1 S45F mutation is likely oncogenic. No real progress has been made in targeting oncogenic mutant forms of CTNNB1 in lung cancer.