New twists on the epigenetics of CpG islands.

CpG islands are >200-bp stretches of DNA that have a significantly higher concentration of CpG dinucleotides than the bulk of the genome. Whereas 70%–80% of all CpG dinucleotides in the human genome are methylated, CpG islands by and large remain unmethylated (Cross and Bird 1995), with the exception of those associated with imprinted and X-linked genes (Razin and Cedar 1994). In this issue of Genome Research, Strichman-Almashanu et al. (2002) present the first successful systematic approach to generating libraries of differentially methylated and unique CpG islands and show its use in uncovering novel imprinted genes.