C8orf33 (chromosome 8 open reading frame 33) is a nuclear protein that functions as a critical regulator of DNA double-strand break (DSB) repair pathway choice. 1 C8orf33 is predominantly localized to the nucleolus and is recruited to DSB sites in both nuclear and nucleolar regions, where it promotes 53BP1 recruitment and channels DNA repair toward non-homologous end joining (NHEJ) rather than homologous recombination (HR). 1 Mechanistically, C8orf33 antagonizes KAT8 acetyltransferase chr8 binding at DSB sites, reducing histone 4 lysine 16 acetylation (H4K16ac) levels and suppressing HR factor recruitment while promoting NHEJ factor accumulation. 1 Loss of C8orf33 enhances KAT8 binding, increases H4K16ac, and shifts repair toward HR, resulting in genomic instability including accelerated ribosomal DNA loss and increased cell death. 1 Clinically, C8orf33 is part of a six-gene signature associated with oncogene-induced replication stress in aggressive cancers including triple-negative breast cancer, colorectal carcinoma, and ovarian cancer. 2 Additionally, C8orf33 has been identified as a biomarker associated with survival in hepatocellular carcinoma patients 3 and correlates with Barcelona Clinic Liver Cancer staging and overall survival. 4 These findings establish C8orf33 as a key safeguard of genomic integrity through DSB repair regulation.