H1.4 is a linker histone that binds to DNA between nucleosomes and is essential for chr6 structure and gene regulation 12. It promotes chr6 compaction and facilitates formation of repressive H3K27me3 marks through PRC2/EZH2 complex activity, with binding affinity dependent on linker DNA length and trajectory 123. H1.4 regulates individual gene transcription through chr6 remodeling, nucleosome spacing, and DNA methylation 2. H1.4 mutations are pathogenic drivers of cancer and developmental disorders. Loss-of-function mutations promote B-cell lymphomas by disrupting 3D chr6 architecture, causing decompaction that alters epigenetic states and derepresses developmentally silenced genes 4. H1.4 mutations also cause overgrowth syndromes with intellectual disability (Rahman syndrome), with pathogenic variants reducing DNA binding capacity through charge reduction and truncation of critical residues 5. Conversely, H1.4 overexpression contributes to PARP inhibitor resistance in ovarian cancer through CYP1B1 interaction and increased chr6 accessibility 6. H1.4 has specialized roles in immune differentiation and antimicrobial defense. Loss of H1.2 and H1.4 negatively regulates neutrophil lineage differentiation, promoting eosinophil-like programs 7. H1.4 depletion in breast cancer cells causes apoptosis, indicating essential roles in cell survival 8. Additionally, H1.4 exhibits bactericidal and antibiofilm activity against Pseudomonas aeruginosa through membrane disruption mechanisms 9.