Bald laboratory mutant mouse (Mus musculus). Such mutant mice have a genetic mutation that results in hairlessness and an absent thymus gland. The thymus produces mature T-cells (a type of immune system cell), which attack foreign tissue and regulate other immune system cells. The lack of a thymus gland means that this mouse will not reject any foreign tissues implanted in its body, allowing researchers to observe the development of diseases such as cancers, and the effect of drugs upon them.