ESR1 is frequently silenced by CGI (CpG island) methylation, both in human colorectal tumours and, in an age-dependent manner, in healthy mucosa. It is not clear, however, whether methylation of individual cytosines occurs randomly within the epithelial genome, or preferentially within individual cells as an 'all-or-nothing' phenomenon. CGI methylation can be quantified in human DNA residues recovered from faecal samples. We used bisulphite genomic sequencing of human DNA from this source and from a colorectal cancer cell line (SW48) to show that the ESR1 CGI is methylated in an allele-specific manner. This provides support for the 'all or none' mechanism for methylation of this gene, and shows how age-dependent methylation of the ESR1 CGI leads rapidly to silencing of the gene within the cells, and hence the colonic crypt within which it occurs. Preliminary studies with a rodent model suggest the rate of age-dependent methylation of ESR1 is modifiable by dietary folate.