HistoneH3 (1-21) is a truncated form of the HistoneH3 protein consisting of amino acids 1 to 21. It serves as a common substrate for methyltransferase assays targeting Histone 3 at lysine 4 and lysine 9, as well as for acetyltransferase assays targeting Histone 3 at lysine 9 and lysine 14.