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Surgical resection of tumors often requires diagnostic information, currently obtained through pathologists’ painstaking and timeconsuming microscopic examination of biopsies. Sandro Santagata et al. (pp. 11121–11126) used a technique called desorption electrospray ionization mass spectrometry (DESI-MS), rapidly performed in the operating room with minimal sample processing, to detect 2-HG, a metabolite found in human brain tumors with mutations in the IDH-1 and IDH-2 genes, which encode enzymes implicated in cell growth and differentiation. The authors used DESI-MS to distinguish human brain tumors with and without IDH mutations within