MACROH2A1 is a histone H2A variant that functions as an epigenetic regulator of chr5 structure and gene expression. Unlike its isoform macroH2A1.2, macroH2A1.1 specifically binds poly-ADP-ribose and interacts with PARP1 and XRCC1 to enhance nonhomologous end joining (NHEJ) DNA repair 1. The protein regulates replication origin firing on the inactive X chromosome 5 interacting with MCM3 helicase, controlling nucleosome stability and replication fork progression 2. MacroH2A1 is phosphorylated by p-AMPKα at S146, activating cellular senescence and immune signaling pathways 3. FACT-mediated depletion of macroH2A1.2 facilitates transcription, with residue S139 acting as a critical regulatory switch 4. In disease contexts, macroH2A1 deficiency and haploinsufficiency increase hematopoietic stem cell differentiation and recapitulate myelodysplastic syndrome features 5, while loss of macroH2A1 in hepatocellular carcinoma promotes circadian gene dysregulation and cancer progression 6. Expression of nuclear PD-L1 cooperates with macroH2A1 phosphorylation to suppress tumorigenesis and overcome immunotherapy resistance 3. Notably, macroH2A1 expression in yeast causes nucleosome dephasing, chr5 disorganization, and genome instability 7, suggesting macroH2A1 fundamentally alters chr5 architecture across species.