This sequence is amino acids 1 to 21 fragment of the histone 4. Histone acetyltransferase (HAT) p300/CBP catalyses the acetylation of this N-terminal tail of free histone H4 at Lys5, 8, 12, 16. Lys8 is thought to be the preferred acetylation site in H4. Determinants of interaction of p300 with histone 4 appear to be fully present on H4-21, the N-terminal region of the protein.