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. Mainly localizes in heterochromatin. Dysplays a punctuate staining pattern in the nucleus. The C-terminal domain is required for high-affinity binding to chromatin. 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. ADP-ribosylated on Ser-55, Ser-113 and Ser-150 in response to DNA damage. H1 histones are progressively phosphorylated during the cell cycle, becoming maximally phosphorylated during late G2 phase and M phase, and being dephosphorylated sharply thereafter. Acetylated at Lys-26. Deacetylated at Lys-26 by SIRT1. Hydroxybutyrylation of histones is induced by starvation. 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. This proves at least that a correct stoichiometry of linker histone deposition on chromatin is essential. Belongs to the histone H1/H5 family.