CCNI (cyclin I) is an atypical cyclin that functions as a regulatory partner for cyclin-dependent kinases, primarily CDK5 and CDK6, controlling cell cycle progression and cardiovascular homeostasis. In its canonical role, CCNI activates CDK5-mediated phosphorylation of retinoblastoma protein (Rb), thereby regulating the Rb/E2F1/DHFR signaling pathway 1. CCNI also forms an active complex with CDK6 to promote E2F-mediated gene expression and cell proliferation, with effects comparable to cyclin D upregulation in breast cancer cells 2. Mechanistically, endothelial CCNI protects against vascular dysfunction by maintaining Rb phosphorylation and supporting biopterin production, which prevents eNOS uncoupling and reduces reactive oxygen species generation 1. The protein localizes to multiple cellular compartments including the nucleus, cytoplasm, and Golgi apparatus, where it regulates G1/S transition and spermatogenesis 3. Clinically, CCNI downregulation in abdominal aortic aneurysm (AAA) patients correlates with increased disease risk, as endothelial CCNI knockout exacerbates hypertension-induced vascular remodeling and AAA formation in mice 1. CCNI upregulation drives breast cancer proliferation and correlates with high E2F target gene expression in cancer tissues 2. Additionally, RNA editing of CCNI (R75G) associates with severity and complications in Mycoplasma pneumoniae pneumonia 4, suggesting CCNI involvement in immune regulation and infection response.