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). Interacts with DFFB. Mainly localizes in euchromatin. Restricted to thymus, testis and spleen. Present also in lymphocytic and neuronal cells. Increases in testis starting with a low level at day 5 and reaching high concentrations in 20-day old and adult animals. 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. Citrullination at Arg-56 (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. Deficient-mice developed normally until the adult stage. No anatomic abnormalities are detected, mice are fertile and they show normal spermatogenesis and testicular morphology. The lack of phenotype may be due to a compensatory function of other histone H1 subtypes. Belongs to the histone H1/H5 family.