PTPN4 (protein tyrosine phosphatase non-receptor type 4) is a non-receptor protein tyrosine phosphatase that functions primarily as an anti-apoptotic regulator and immunomodulatory protein. The protein consists of two globular domains - a PDZ domain and a phosphatase domain - connected by a flexible linker that regulates phosphatase activity 1. PTPN4 protects neurons against apoptosis by regulating neuronal cell homeostasis 1. In immune signaling, PTPN4 negatively regulates TLR4-induced interferon beta production by dephosphorylating adapter TICAM2, thereby inhibiting TRAM-TRIF interactions 1. The protein shows tumor suppressor activity in multiple cancers - it is inversely associated with multiple myeloma risk 2, and its degradation by E3 ubiquitin ligase MARCH8 promotes pancreatic cancer growth and metastasis through STAT3 activation 3. PTPN4 also serves as a target for high-risk HPV E6 oncoproteins, which bind to its PDZ domain potentially contributing to viral pathogenesis 4. Additionally, circRNA-PTPN4 regulates blood-brain barrier integrity and cognitive function in uremic encephalopathy by modulating FOXO3 and ZO-1 expression 5. These findings highlight PTPN4's diverse roles in neuroprotection, immune regulation, and cancer suppression.