One of the most challenging problems in eukaryotic genome biology is to elucidate the relationship between chromatin structure and gene activity. The metazoan genome is a complex structure in which DNA and its associated proteins, in particular the histones, form the nucleosomes — the fundamental repeat subunit of chromatin. 1 , 2 Although core histones are essential for the structural integrity of the nucleosomes, it is becoming increasingly clear that chromatin-associated nonhistone proteins may critically affect the structure-function relationships of nucleosomes in chromatin. For recent reviews see Mathis et al. 3 and Cartwright et al. 4