PIAS4 (protein inhibitor of activated STAT 4) functions primarily as an E3-type small ubiquitin-like modifier (SUMO) ligase that catalyzes the conjugation of SUMO proteins to diverse target substrates 1. The protein mediates SUMOylation of numerous cellular targets including PARP1, TDP-43, PCNA, SLC7A11, FOXK2, AMPKα, and NEMO, regulating their stability, localization, and activity 123456. PIAS4 plays critical roles in DNA damage response by SUMOylating PARP1 following chr19 trapping, facilitating its removal by the p97 ATPase complex 1. The protein protects against protein aggregation by SUMOylating TDP-43 during oxidative stress, preventing irreversible aggregation in stress granules 2. In DNA damage tolerance pathways, PIAS4 promotes error-free template switching over error-prone translesion synthesis through PCNA SUMOylation at Lys164 3. Disease relevance includes hepatocellular carcinoma, where PIAS4 upregulation correlates with poor prognosis and promotes tumorigenicity through SUMOylation of metabolic regulators 6. Additionally, PIAS4 contributes to chemotherapy resistance by regulating nucleotide synthesis pathways 5 and ferroptosis sensitivity through SLC7A11 SUMOylation 4. Genomic duplications affecting PIAS4 regulation cause proximal 19p13.3 microduplication syndrome with developmental abnormalities 7.