A stable, fluorescently labeled zebrafish melanoma cell line derived from transgenic mitfa-BRAF^V600E;p53^−/−;mitfa-GFP fish was transplanted into casper, a transparent adult zebrafish. Green fluorescent melanoma cells metastasize to various regions of the transparent host fish over time. Shown here are sequential images of a tumor transplant that enable the quantitative assessment of metastatic spread in vivo. For details, see article by Heilmann and colleagues on page 4272.