Seeing the sugar coating: N-Acetyl-glucosamine and mannosamine derivatives tagged with an isonitrile group are metabolically incorporated into cell-surface glycans and can be detected with a fluorescent tetrazine. This bioorthogonal isonitrile-tetrazine ligation is also orthogonal to the commonly used azide-cyclooctyne ligation, and so will allow simultaneous detection of the incorporation of two different sugars.