<p>CpG methylation is uniform within the normal cellular hierarchy of the intestine, and PRC2-associated H3K27me3 marks are present in crypt and villus cells (blue, to the left). Upon tumour initiation, recurring CpG methylation patterns form, guided by an instructive mechanism that is linked to PRC2 for hypermethylated sites (blue to green). Further CpG methylation changes occur slowly, probably in a stochastic manner. A fraction of these bestow tumour cells with a selective advantage and are subject to clonal expansion during tumour progression (green to red).</p
