Since the 1960s, scientists have been able to breed mice that have deficient immune systems with no functional T-or B-cells. Human cancer cells that are implanted into these mice are not rejected, as they would be in mice with normal immune systems. Thus, the natural history of untreated human cancers, as well as the impact of various treatments can be studied in mice.

	Nude Mice: Hairless mutant mice that are immune deficient were developed in 1937. They do not reject tumor transplantations from other species, allowing actual human tumors to be studied in a whole animal system.

• SCID Mice: Mice with severe combined immune deficiency (SCID) were discovered in 1983. SCID mice are even more immune deficient than nude mice. Tumors from other species are easily transplanted into SCID mice and will grow without being rejected.

Xenografts & Mouse Models

The use of xenografts in mouse models has led to much better prediction about how cancer actually behaves in humans.

Tissue or organs from an individual of one species is transplanted into or grafted onto an organism of another species, genus, or family. So in this picture you see a mouse that had cells from a human tumor injected into it. The cells grew into a tumor in the mouse and then the mouse was given a new drug to observe the effect.

Still, many new treatments that appear quite effective in these models either don't work in humans, or prove to be too toxic.