INPP5J (inositol polyphosphate-5-phosphatase J) is a lipid phosphatase that catalyzes the removal of 5-phosphate from phosphoinositides, converting phosphatidylinositol 3,4,5-trisphosphate (PtdIns(3,4,5)P3) to phosphatidylinositol 3,4-bisphosphate (PtdIns(3,4)P2) 1. This enzymatic activity suppresses PI3K/AKT signaling, a key oncogenic pathway 2. INPP5J functions as a tumor suppressor across multiple cancer types. In pancreatic cancer, INPP5J expression is markedly reduced, and its overexpression inhibits cell proliferation, invasion, and migration; conversely, INPP5J knockdown promotes these malignant behaviors 3. In breast cancer, INPP5J (also called PIPP) cooperatively suppresses AKT signaling with PTEN, and combined loss of both phosphatases is associated with enhanced cell proliferation and reduced patient survival 4. In ovarian cancer, the microRNA miR-661 promotes proliferation by directly repressing INPP5J expression and subsequently activating AKT signaling 5. INPP5J degradation can be promoted through ubiquitination by E3 ligase PELI1, linking protein stability regulation to cancer progression 3. Additionally, INPP5J has been identified as a potential tumor antigen in lung adenocarcinoma, with expression correlating to immune cell infiltration patterns 6. Its dysregulation appears central to multiple malignancies.