a technique to measure genome-wide methylation levels of single cytosines by treating the DNA with sodium bisulfite, which converts unmethylated cytosines into uracil while methylated cytosines remain unchanged. After sequencing, the unmethylated cytosines appear as thymines and methylated cytosines are still read as cytosines. This method provides single-base resolution of DNA methylation throughout the genome. However, it requires essentially resequencing the entire genome multiple times for every experiment. To perform WGBS on large genomes (e.g., mammalian genomes) is very expensive to achieve the 10x coverage requirement.
Reference: Lister, R., et al., Human DNA methylomes at base resolution show widespread epigenomic differences. Nature, 2009. 462(7271): p. 315-22.