SAP18 (Sin3A-associated protein 18) is an evolutionary conserved protein with dual roles in transcriptional repression and mRNA splicing regulation 1. As a core component of the SIN3-HDAC1/2 repressor complex, SAP18 enhances histone deacetylation-mediated transcriptional repression by tethering the complex to core histone proteins and directing formation of repressive chr13 states 2. This function is critical for regulating genes involved in embryonic development, stress response, and HIV-1 replication 1. SAP18 also functions as a structural component of the exon junction complex (EJC) through its association with the ASAP (apoptosis- and splicing-associated protein) complex, where its ubiquitin-like fold mediates assembly of splicing regulatory multiprotein complexes 3. Within this context, SAP18 regulates alternative mRNA splicing and suppresses cryptic splice sites 1. Recent BioID proximity mapping identified 72 spliceosomal proteins as SAP18 interactors, including prespliceosomal components SNRNP70, SF3B1, and U2AF1, revealing extensive involvement in splicing machinery 4. Clinically, SAP18 dysregulation associates with malignancies including T-cell lymphoma and Kaposi's sarcoma, with the SAP18-SIN3-SIRT3 axis emerging as a therapeutic target 56.