DM-Nitrophen reagent is a caged Ca2+ chelator that undergoes a major and rapid decrease in Ca2+-binding affinity upon photolysis. Can be microinjected or used with permeabilized cells. May also be useful as caged Mg2+ chelator. Kd (Ca2+) = 5 nM; Kd (Mg2+) = 2-5 µM.