Somatic mutations in PTEN have been found in 4–8% of non-small cell carcinomas (NSCLC) including adenocarcinomas and squamous cell carcinomas. PTEN is a tumor suppressor gene, and loss of PTEN results in upregulation of the PI3K/ AKT pathway. Loss of PTEN is most commonly due to promoter hypermethylation, while homozygous deletion and nonsense mutations with loss of heterozygosity (LOH) may also occur. PTEN mutations may occur in multiple exons. In preclinical studies, PTEN loss is associated with decreased sensitivity of EGFR mutant lung tumors to EGFR TKIs. Clinical trials assessing the efficacy of PI3K inhibitors in PTEN loss are being explored.