An image of a tube formation assay taken with an epifluorescence microscope. The tube formation assay is a 3D in vitro assay that mimics the formation of vasculature in vivo. NCI’s Dr. Enrique Zudaire is using this assay to discover new drugs to block tumor angiogenesis and tumor growth. The nuclei of endothelial cells appear blue. In green are the endothelial cells, which have been genetically engineered to express a green fluorescent protein. The thinner areas are hollow tubes while the thicker areas are cell clusters.