Estradiol Glow is the ovarian steroid hormone 17-beta-Estradiol labeled with a novel low molecular weight orange/red fluorophor thus retaining its chemical properties and its biological activity. This allows for a wide range of applications includinganalysis of in vivo and in vitro steroid uptake in real timemonitoring of intracellular and subcellular steroid transportstudies on steroid-binding proteinsuse as tracer for steroid immunoassays (to replace isotopes or enzymesphotodynamic elimination of estrogen sensitive tissues and cells in experimental settings with possible future clinical implicationsEstradiol Glow can be histologically fixed using formaldehyde or glutaraldehyde without any significant loss of fluorescence intensity. Typical concentration range used for cell culture experiments: 10-5M - 10-7M (may vary depending on cell line and individual experimental set-up). Background Identification of steroid sensitive cells in target tissues has so far been attempted with either radio labelled hormones and autoradiography or with in situ hybridization and immuno-histochemistry for receptor proteins. Numerous studies have been performed with steroid hormones labelled with either enzymes, fluorophors or immunogenic markers, all resulting in loss of biological properties and hormonal function. Estradiol Glow is 17-beta-estradiol, labelled in a novel way with a low molecular weight biocompatible fluorophor, thus retaining biological activity of the ovarian hormone. Systemic injections into experimental animals revealed accumulation of the fluorescent steroid in nuclei of known estrogen target tissues, including the brain. Estradiol Glow allows new insights into cellular uptake, intracellular transport and subcellular binding. Mechanisms of rapid steroid actions become transparent on the single cell level.