– Histone H3 (21-44)-GK-biotin is a peptide fragment of histone H3 that corresponds to amino acid residues 22-45 of the human histone H3.3 sequence and is biotinylated via a C-terminal GK linker. Unlike histone H3.1 and H3.2, the histone H3.3 variant contains a serine residue at position 31 that is phosphorylated during late prometaphase and metaphase of mitosis. Histone H3 (21-44) also contains lysine residues at positions 23, 27, and 36 that are subject to methylation and acetylation, all of which have a role in the regulation of gene expression, and a serine residue at position 28 that is subject to phosphorylation during mitosis.