The lipid phosphatase PTEN, which converts the membrane lipid phosphatidylinositol (3,4,5)-trisphosphate (PtdIns(3,4,5)P3) to phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P2), is the second most commonly mutated gene in human cancers. The gene discussed is PTEN; the disease is cancer.