Nucleosides that form human transfer RNA, messenger RNA, and an assortment of microRNAs are decorated with about 100 different posttranslational modifications, from added methyl groups to added sugars. These modifications serve a variety of functions, such as helping a tRNA maintain its three-dimensional structure and recognize the amino acid it is meant to carry as well as where to deliver it. Despite widespread distribution of modified nucleosides in humans, researchers haven’t been able to quantify the number of individual modified bases in a cell. Now, a team led by Thomas Carell of Ludwig Maximilians University, in Munich, Germany, has developed a proteomics-based approach that aims to do just that (Angew. Chem. Int. Ed., 10.1002/anie.200902740). The researchers first synthesized deuterated versions of six modified nucleosides, including isopentenyladenosine (i6A), and then spiked cell extracts with the compounds. By comparing the mass spectrometry peaks of the known amount of deuterated nucleoside with those of natural nucleosides in the sample, the team determined the number of modifications. To test the method, they compared the levels of modified nucleosides in cancerous versus normal mouse cells, observing changes in the amounts of several nucleosides.