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 H3.1 and 3.2 sequences and is biotinylated via a C-terminal GK linker. Histone H3 (21-44) contains a lysine residue at position 23 that is subject to acetylation, an arginine at position 26 subject to methylation, and a serine at position 28 subject to phosphorylation, as well as lysine residues at positions 27 and 36 that are subject to methylation and acetylation. Histone H3 (21-44)-GK-biotin has been used as a substrate for the primate-specific histone methyltransferase PR domain-containing protein 7 (PRDM7) to determine substrate specificity.