ZNF639 (zinc finger protein 639), also known as ZASC1, is a Krüppel-like zinc finger transcription factor that functions as both a transcriptional activator and repressor depending on cellular context 1. As a sequence-specific DNA-binding protein, ZNF639 contains zinc fingers that mediate protein-protein interactions and DNA binding 2. ZNF639 interacts with alphaN-catenin and recruits it to the nucleus, functioning as a transcriptional repressor 1. In viral contexts, ZNF639 activates murine leukemia virus (MLV) and HIV-1 transcription by recruiting the elongation factor P-TEFb and viral transactivator proteins to promoter regions 34. ZNF639 is frequently amplified at chromosome 3 in multiple squamous cell carcinomas (esophageal, oral) and associates with tumor progression and lymph node metastasis 256. Conversely, in breast cancer, reduced ZNF639 expression correlates with poor prognosis; ZNF639 knockdown increases clonogenicity, cell motility, and lung metastasis through upregulation of epithelial-mesenchymal transition markers including Snail1, vimentin, and CCL20 7. Thus, ZNF639 exhibits context-dependent tumor suppressive and oncogenic roles, representing a novel prognostic determinant across multiple cancer types.