DMNPE-4 AM-caged-calcium, a photolabile analogue of EGTA, is a highly efficient Ca2+-selective cage. At pH 7.2 and pH 7.4, its Kd for Ca2+ is measured at 48 nM and 19 nM, respectively. Upon photolysis, DMNPE-4 AM-caged-calcium exhibits a decreased affinity for Ca2+ (~2 nM Kd).