Experimental mice. Two nude mice, Mus muscula, which have been specially bred for use in medical experiments. These mice does not have a functioning thymus gland, meaning that some of the cells in their immune systems cannot mature. These T cells would normally attack foreign tissue and regulate the activity of other types of immune cells. Due to their deficiency, these mice can have samples of cancer-causing chemicals (carcinogens), human tumours or human tissues transplanted into their bodies. The development of any cancers, and the effect of drugs upon them, can then be observed.