Two modifications of cytosine, one of the four bases that make up DNA, look almost the same but mean different things. But scientists lacked a way of reading DNA, letter by letter, and detecting precisely where these modifications are found in particular tissues or cell types. The team used the technique to map 5-methylcytosine (5-mC) and 5-hydroxymethylcytosine (5-hmC) in DNA from human and mouse embryonic stem cells, revealing new information about their patterns of distribution. These studies have revealed that these DNA modifications play major roles in fundamental life processes such as cell differentiation, cancer and brain function. "They regulate gene expression and have a broad impact on stem cell development, various human diseases such as cancer, and potentially on neurodegenerative disease. They may even shape the development of the human brain.