Background: Changes in chromatin structure play a large role in the regulation of transcription in eukaryotes (1). The nucleosome is the primary building block of chromatin, and is made up of four core histone proteins (H2A, H2B, H3 and H4) (2). Acetylation of histone H4 thus appears to play a primary role in the structural changes that mediate enhanced binding of transcription factors to their recognition sites within nucleosomes (3). Histone H4 can be reversibly acetylated at lysine residues 5, 8, 12 and 16. It has been shown that, in H4 from human cells, the four lysine residues are acetylated in a preferred, but not exclusive order, namely lysine 16, followed by 12 and 8, followed by 5 (4).