Hugh Pickens writes "What is transparent, swims, and helps cure cancer? Caspar the friendly fish — a zebrafish bred with a see-through body to make studying disease processes easier for rapidly changing processes such as cancer, Zebrafish are genetically similar to humans in many ways and serve as good models for human biology and disease. In one experiment, researchers inserted a fluorescent melanoma tumor into the abdominal cavity of the transparent fish and by observing the fish under a microscope, they found that the cancer cells started spreading within five days and could actually see individual cells spreading. "The process by which a tumor goes from being localized to widespread and ultimately fatal is the most vexing problem that oncologists face," says Richard White, a clinical fellow in the Stem Cell Program at Children's Hospital Boston. "We don't know why cancer cells decide to move away from their primary site to other parts in the body." Researchers created the transparent fish, (photo) by mating two existing zebrafish breeds, one that lacked a reflective skin pigment and the other without black pigment. The offspring had only yellow skin pigment, essentially appearing clear."