Protein phosphorylation, which is a major mechanism of cell signaling, can occur only on certain amino acids, including serine. Peter G. Schultz and coworkers at Scripps Research Institute report that they can control protein phosphorylation on serine residues by genetically incorporating a photoprotected serine analog (shown) into proteins (Nat. Chem. Biol., DOI: 10.1038/nchembio.2007.44). Phosphorylation can't occur at the protected serine residues. But shining blue light on the protein removes the 4,5-dimethoxy-2-nitrobenzyl protecting group, thereby exposing serine and making it available for phosphorylation. The researchers used the serine analog to regulate phosphorylation of specific serine residues in Pho4, a yeast transcription factor that regulates genes and allows the organism to grow at different inorganic phosphate concentrations. By replacing one serine residue at a time with the photoprotected analog, the team was able to observe the effects of blocking phosphorylation at specific sites and measure dynamic responses to the phosphorylation event.