CCNH (cyclin H) is a regulatory subunit of the CDK-activating kinase (CAK) complex that functions in both cell cycle control and transcription 1. As a component of the CAK complex with CDK7 and MAT1, CCNH activates cyclin-dependent kinases (CDK1, CDK2, CDK4, CDK6) through threonine phosphorylation, regulating G1/S cell cycle progression 1. Additionally, when associated with the TFIIH basal transcription factor complex, CCNH participates in RNA polymerase II activation by facilitating serine phosphorylation of the CTD domain, enabling transcriptional elongation 1. CCNH expression and activity remain constant throughout the cell cycle, localized primarily in the nucleoplasm 1. Disease relevance includes congenital microcephaly, where CCNH mutations have been proposed as a candidate cause 2. Clinically, elevated CCNH expression correlates with poor prognosis in lung cancer, where it promotes cell proliferation, invasion, and migration 3. CCNH dysfunction also affects DNA damage repair pathways, as pathogenic fungal toxins (candidalysin) target CCNH to promote double-strand DNA breaks and impair DNA repair responses 4. These findings position CCNH as both a promising biomarker and therapeutic target in cancer treatment.