Illustration of bisulfite genomic sequencing of the coding strand (exon 3) of the ρ-globin gene.

Arrows indicate cytosine residues at CpG dinucleotides. These cytosines have been completely converted to thymidines (indicating unmethylated cytosines) in DNA from day 4 embryonic erythroid cells. Progressive failure of conversion to thymidines (indicative of methylation) is seen during development, and DNA derived from adult erythroid cells show completely methylated cytosines. Cytosine residues not associated with CpG dinucleotides (indicated by an asterisk) have been completely converted to thymidines in all samples.