PTEN (phosphatase and tensin homolog) is a tumor suppressor gene that prevents cells from growing and dividing too rapidly and out of control. PTEN removes phosphate groups from a molecule called PIP3 to generate PIP2. This dephosphorylation helps to ensure the inhibition of the AKT cell survival pathway. PTEN protein is found in all types of cells, and loss of PTEN function is one of the most common occurrences in multiple advanced human cancers. PTEN is an important mediator in signaling downstream of HER-family receptors which can be blocked by EGFR- and HER2-targeted therapies. Loss of PTEN function is associated with a lack of response to these therapies.