Histone H1 protein binds to linker DNA between nucleosomes forming the macromolecular structure known as the chromatin fiber. Histones H1 are necessary for the condensation of nucleosome chains into higher-order structured fibers. Acts also as a regulator of individual gene transcription through chromatin remodeling, nucleosome spacing and DNA methylation (By similarity). Mainly localizes in euchromatin. The C-terminal domain is required for high-affinity binding to chromatin. H1 histones are progressively phosphorylated during the cell cycle, becoming maximally phosphorylated during late G2 phase and M phase, and being dephosphorylated sharply thereafter. Crotonylation (Kcr) is specifically present in male germ cells and marks testis-specific genes in post-meiotic cells, including X-linked genes that escape sex chromosome inactivation in haploid cells. Crotonylation marks active promoters and enhancers and confers resistance to transcriptional repressors. It is also associated with post-meiotically activated genes on autosomes. ADP-ribosylated on Ser-187 in response to DNA damage. Citrullination at Arg-54 (H1R54ci) by PADI4 takes place within the DNA-binding site of H1 and results in its displacement from chromatin and global chromatin decondensation, thereby promoting pluripotency and stem cell maintenance. Hydroxybutyrylation of histones is induced by starvation. No visible phenotype. Triple-deficient mice (H1-2, H1-3 and H1-4) die by midgestation with a broad range of defects. These embryos have about 50% of the normal ratio of H1 to nucleosomes, demonstrating that critical levels of total H1 histones are essential for mouse embryogenesis. Belongs to the histone H1/H5 family.