H2A.Z1 (H2A.Z variant histone 1) is a histone variant that replaces conventional H2A in nucleosomes, functioning as a central regulator of chr4 architecture and gene expression. As a structural chr4 component, H2A.Z1 associates with functional regulatory elements including promoters, enhancers, and insulators 1. Its C-terminal tail is critical for nucleosome stability and chr4 organization, with tail-dependent mechanisms influencing global chr4 accessibility and gene expression 2. H2A.Z1 plays distinct roles in DNA replication licensing and activation; nucleosomes containing H2A.Z1 facilitate origin-recognition complex binding and early replication origin firing through the SUV420H1-H4K20me2-ORC1 axis 3. In intestinal epithelium, H2A.Z1 antagonistically regulates differentiation with its isoform H2A.Z.2, preventing terminal enterocyte differentiation while supporting stem cell maintenance 4. Clinically, H2A.Z1 dysregulation associates with multiple pathologies: deficient H2A.Z deposition drives uterine leiomyoma tumorigenesis through epigenetic instability 5, while H2A.Z1 overexpression correlates with poor prognosis in hepatocellular carcinoma via enhanced tumor stemness 6. Germline H2AZ1 variants cause rare neurodevelopmental disorders 7.