CCNJ (cyclin J) is a cell cycle regulatory protein functioning as a cyclin-dependent protein kinase regulator involved in G1/S transition and mitotic cell cycle progression 1. CCNJ localizes to the cytoplasm, nucleus, and microtubule organizing centers, where it regulates cyclin-dependent serine/threonine kinase activity. The protein controls cell mitosis and proliferation through its role in cell cycle checkpoint regulation 2. CCNJ is dysregulated across multiple human malignancies through distinct mechanisms. In hepatocellular carcinoma (HCC), CCNJ promoter hypermethylation (78.8% of tumors) leads to decreased expression and correlates with worse overall survival 1. In breast cancer, CCNJ is overexpressed in ~20% of tumors and serves as a direct target of the tumor suppressor miR-125b 3. Similarly, CCNJ is targeted by miR-125a in HCC cells 4, miR-16 in ErbB-2-positive breast and gastric cancer 5, miR-146a in cisplatin-resistant lung cancer 6, and miR-98 in prostate cancer 2. Upregulation of these regulatory miRNAs suppresses CCNJ expression, inhibiting cell proliferation and enhancing drug sensitivity. Clinically, CCNJ represents a promising prognostic marker and therapeutic target in HCC and other cancers, with potential utility in predicting chemotherapy response and identifying candidates for miRNA-based therapeutics.