Publisher Summary This chapter presents four histone modifications and their functions in chromatin structure and function. The four core histones—H2A, H2B, H3, H4—and their variants and the linker histone H1 subtypes are susceptible to a wide range of postsynthetic modifications, including acetylation, phosphorylation, methylation, ubiquitination, and ADP-ribosylation. The phosphorylation of the core histones has been implicated in transcription, replication, chromosome condensation, and DNA repair. Cell-cycle studies of histone phosphorylation using synchronized Chinese hamster ovary cells and HeLa S-3 cells demonstrated that H1 and H3 are phosphorylated at different times during the cell cycle, while H2A and H4 are phosphorylated at uniform rates throughout the cell cycle. Kinetic studies of the phosphorylation of H2A and H4 in trout testis indicate that these histones are phosphorylated shortly after synthesis. The phosphorylation of H4 did not occur appreciably until after a series of acetylation and deacetylation events, while H2A was phosphorylated shortly after the synthesis followed by dephosphorylation. Studies with native and reconstituted chromatin show that phosphorylated H1 destabilizes a chromatin structure. The phosphorylation of H1 increases the protein's mobility in the nucleus and weakens its interaction with chromatin.