PTEN is an obligate haplo-insufficient tumor suppressor gene and is commonly mutated in a large number of cancers. It negatively regulates intracellular levels of Phosphatidylinositol (3,4,5)-trisphosphate (PIP3) in cells and functions as a tumor suppressor by negatively regulating AKT/mTOR signaling pathway. Mono- and bi-allelic loss of PTEN is found in approximately 40-50% and 5% of breast cancers, respectively. It has been reported to occur in BRCA1-associated basal-like breast cancer. Germline mutations in PTEN are also responsible for Cowden disease, a rare autosomal dominant multiple-hamartoma syndrome. In one study, germline mutations of PTEN have been reported to be associated with 85% lifetime risk of breast cancer in patients with PTEN hamartoma tumor syndrome. Aberrant PTEN pathway is associated with metastases and poor prognosis in breast cancer. It also predicts poor response to trastuzumab. There are ongoing clinical trials investigating anti-tumor activity of PI3K-beta inhibitor in PTEN deficient tumors.