API5 (apoptosis inhibitor 5) is a multifunctional nuclear protein that primarily functions as an antiapoptotic factor through multiple regulatory mechanisms 1. API5 inhibits apoptosis by binding to and negatively regulating ACIN1, suppressing ACIN1 cleavage by CASP3 and preventing ACIN1-mediated DNA fragmentation 1. The protein undergoes critical post-translational modifications including acetylation at K251 by p300 histone acetyltransferase, which maintains protein stability, while HDAC1-mediated deacetylation promotes instability and regulates subcellular localization 2. API5 also undergoes SUMOylation at K404, which enhances MDA5-dependent IFN-β production for antiviral immunity 3. Additionally, SRPK1-dependent phosphorylation at S464 prevents autophagic degradation of cytosolic RNA sensors RIG-I and MDA5, promoting antiviral responses 4. Beyond apoptosis regulation, API5 functions in mRNA nuclear export through interaction with nuclear FGF2, regulating bulk mRNA export and specific transcripts like c-MYC and cyclin D1 5. API5 can also be secreted by γδ intraepithelial lymphocytes to protect Paneth cells from death in Crohn's disease models 6. Elevated API5 expression is associated with various cancers and poor prognosis, making it a potential therapeutic target 1.