FIGURE 1.

Definitions and states of CpG islands. (A) Depiction of the typical CpGs in the mammalian genome which are DNA methylated in isolation but are devoid of this mark in the CGI context. CGIs were originally defined bioinformatically. (B) Schematic demonstrating CGIs locations were biochemically determined. MBD and CXXC proteins fixed to a Sepharose column allowed purification of DNA methylated and unmethylated CGIs in mammals. (C) Representation of CGIs across mammalian genomes and a table, summarising the proportion and methylation status of CGIs as reported by Illingworth et al. (2010) in mouse and human across TSS, Intragenic and Intergenic regions. Total CGI numbers in mouse = 23,021, human = 25,495 (D) Summary of the main states of CGIs across the genome. Their active state is associated with binding of transcription factors (TF) and subsequent RNA Polymerase II (Pol II) binding. Repressed states of CGIs are through combinations of DNA methylation, H3K4me3 and H3K27me3.