Geneticists faced a problem with the recent discovery of a "sixth nucleotide" in DNA. It turns out that two modifications of cytosine, one of the four DNA bases, look almost the same but mean different things. Scientists lacked the ability to read DNA precisely enough to detect where these modifications are found. Now, a team of scientists has developed and tested a technique to accomplish this task. 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 information about their patterns of distribution and 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," said Chuan He, professor in chemistry at UChicago.