In mammalian cells DNA is packaged into chromatin to facilitate its organisatin in the nucleus, to protect it from DNA damage, to facilitate nuclear processes such as gene transcription, replication and repair and finally to add a further level of regulation to transcription. The packaging of DNA into chromatin can be considered on 3 different levels primary, secondary and tertiary. Our research group is predominantly working on the secondary level of chromatin fibre folding. The primary level is the folding of DNA around an octamer of histone proteins to form a nucleosome, the second is the folding of the nucleosomes into a 30nm fibre structure and the tertiary level is the levels of folding above this to form the interphase chromatin fibre. To date we have a good understanding of primary chromatin folding and there are now high resolution crystal structures of the nucleosome. In contrast far less is known about the secondary and particularly the tertiary levels of chromatin folding.