A new sialic acid in the house! A biosynthetic approach gives access to previously inaccessible C7 modifications of sialic acids in living cells. Metabolic incorporation of synthetically derived N-acetyl-4-azido-4-deoxymannosamine (see scheme) into glycans of mammalian cells and zebrafish larvae preserves the bioorthogonal functionality of its azido group for subsequent labeling with biophysical probes.