Modifying the surface of cells with synthetic polymers offers a strategy for manipulating cellular behavior. Usually researchers make such modifications by attaching premade polymers to functional groups on the cell surface, but that “grafting-to” approach isn’t very efficient. An alternative approach involves “grafting from,” in which a polymer grows from an initiator on the cell surface. But this has been hard to do in ways that keep the cells—especially mammalian cells—alive. H. Tom Soh of Stanford University; Jia Niu and Craig J. Hawker of the University of California, Santa Barbara; and coworkers report an approach using a light-mediated controlled radical polymerization method that is gentle enough to use with mammalian cells (Nat. Chem. 2017, DOI: 10.1038/nchem.2713). The researchers used cell-surface-anchoring derivatives of 2-(butylthiocarbonothioyl)propionic acid as the chain-transfer agents that are attached to the cells and initiate the reaction. The polymer chain (shown) grows through the addition of cell-compatible polyethylene glycol-based acrylamide monomers. Yeast cells modified this way continued to grow and divide normally. For mammalian cells, which are more delicate, the researchers switched to lipidlike anchor compounds that insert themselves into the cell membrane. They could then modify mammalian cells without harming them. The researchers plan to use the coating to control the cells’ biochemistry and to modulate cell-cell interactions.