A lab rat presses a lever to lift a weight. Animal testing, also known as animal experimentation, animal research, and in vivo testing, is the use of non-human animals in experiments that seek to control the variables that affect the behavior or biological system under study. In the USA the numbers of rats and mice used is estimated to be between 20-100 million a year. Mice are the most commonly used vertebrate species because of their size, low cost, ease of handling, and fast reproduction rate. Mice are widely considered to be the best model of inherited human disease and share 99% of their genes with humans. With the advent of genetic engineering technology, genetically modified mice can be generated to order and can provide models for a range of human diseases. Rats are also widely used for physiology, toxicology and cancer research, but genetic manipulation is much harder in rats than in mice, which limits the use of these rodents in basic science. Animal rights organizations question the need for and legitimacy of animal testing, arguing that it is cruel and poorly regulated or that animals have the intrinsic right not to be used or harmed in experimentation.