H3C2 (H3 clustered histone 2) encodes a core histone protein involved in chr6 structure and gene regulation. As a member of the histone H3 family, H3C2 serves as a substrate for epigenetic modifications, particularly at lysine 27, which plays a critical role in DNA replication regulation 1. In normal physiology, H3C2 expression correlates with metabolic pathways in adipose tissue, with baseline expression predicting weight loss response to lifestyle intervention in women with polycystic ovary syndrome 2. Clinically, H3C2 mutations have emerged as significant disease drivers. H3K27M mutations in H3C2 occur in diffuse midline gliomas (DMGs), an aggressive pediatric brain malignancy, where they associate with worse prognosis and represent a validated biomarker for liquid biopsy monitoring via cerebrospinal fluid and plasma cell-free DNA 3. H3C2 is upregulated in sinonasal undifferentiated carcinoma (SNUC) alongside EZH2, implicating epigenetic dysregulation in tumorigenesis 4. In pediatric pontine high-grade gliomas, H3C2/3 K27M mutations correlate with improved overall survival compared to H3-3A mutations and show distinct magnetic resonance imaging characteristics 5. Additionally, paratesticular rhabdomyomas harbor recurrent H3C2 p.K37I mutations affecting conserved lysine residues 1, and H3C2 mutations associate with poor survival in TMB-low metastatic cancers treated with immune checkpoint inhibitors 6.