In the paper presented at the end of Lezione 14 dicembre (Lister et al., 2009, Nature 462: 315-322), Authors followed a protocol of bisufite treatment of the DNA extracted from either human fibroblasts or embryonic stem cells, which converts cytosine, but not methylcytosine, into uracil and then subjected the DNA to deep sequencing.
The Authors say that they did a total of 1.16 and 1.18 "billion" reads, that corresponds to 178 Gigabases of sequence (something like resequencing 57 times the entire human genome !!!) in order to map at nucleotide resolution the methylcytosine content in these cells.
Could you get any explanation why so much sequencing is required to achieve this result? If you can not guess, just take a look to the accompanying "News & views" article in the same issue of Nature (Schubeler 2009, Nature 462: 296-297).