The initial event of transcription of any gene requires the formation of a pre-initiation complex which associates with the gene’s promoter region. Once activation of this complex has occurred, the RNA polymerase leaves the promoter region and begins the process of elongation of nascent RNA chains. This is the picture that has emerged from countless studies performed with purified transcription factors, purified polymerase and purified DNA as a template. But in cells DNA is not free, it is wrapped around nucleosomes (which are octamers of 4 different histones). This association is unfavorable to the process of transcription and must be altered in order for the pre-initiation complex to access promoter regions, i.e. in order for transcription to start.