Mammalian CpG islands
	areas of high CG concentration, which is generally underrepresented in the genome 1-2 kb; tend to be unmethylated, have reduced contents of histone H1 with acetylation of the other histones and to be sensitive to nucleases frequently present upstream of housekeeping genes; only a minority of tissue regulated genes have them methylation may be acquired by cell lines-> can't express normal cellular functions