Through cross-breeding, scientists have created a zebra-fish with a transparent skin so that they can actually see the insides of the fish. Zebra fish are genetically similar to humans in a number of ways, and hence serve as good subjects for experiments like this. These are now being used to understand cancer. Excerpt:
White created the transparent fish, which he nicknamed Casper, 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.
In one experiment, White and his colleagues inserted a fluorescent melanoma tumor into the abdominal cavity of the transparent fish. By observing the fish under a microscope, they found that the cancer cells started spreading within five days. White could actually see individual cells spreading.
See full article. They also have a picture of the fish for you to ogle.