The green-fluorescent BODIPY® FL-X thapsigargin has spectral properties similar to that of fluorescein and may be useful for localization of thapsigargin binding sites in cells. Thapsigargin is a naturally occurring sesquiterpene lactone isolated from the umbelliferous plant Thapsia garganica. This tumor promoter releases Ca2+ from intracellular stores by specifically inhibiting the endoplasmic reticulum Ca2+-ATPase; it does not directly affect plasma membrane Ca2+-ATPases, Ins 1,4,5-P3 production or protein kinase C activity.