A method to follow the progression of metastasis of a primary tumor, which method comprises optionally removing fresh organ tissues from a vertebrate subject which has been modified to contain tumor cells that express GFP and observing the tissues in situ or observing the excised tissues for the presence of fluorescence is disclosed. Vertebrate subjects which contain GFP producing tumors are useful models to study the mechanism of metastasis. In addition, subjects already harboring tumors can be treated so as to modify the endogenous tumors to contain GFP. This permits clinical applications. Finally, by injecting a contrast dye into a subject harboring a GFP-labeled tumor, angiogenesis in the tumor can be observed directly.