The FAT4 protein, a member of the cadherin-associated protein family, primarily functions as a tumor suppressor by inhibiting cell proliferation and metastasis. FAT4 inhibits cell proliferation and metastasis by regulating the activity of the PI3K/AKT/mTOR and PI3K/AKT/GSK-3B signaling pathways. It is often found inactivated in various human cancers, including melanoma, pancreatic cancer, gastric cancer, and hepatocellular carcinoma. FAT4 also plays a role in regulating Planar Cell Polarity (PCP), which is important for proper tissue organization and development of organs like the kidney. Mutations in FAT4 can lead to cystic kidney disease in mice, suggesting a link between PCP and kidney development.