Animals such as the fruit fly Drosophila melanogaster, the zebrafish, the chicken and the house mouse, serve a major role in science as experimental models,[19] both in fundamental biological research, such as in genetics,[20] and in the development of new medicines, which must be tested exhaustively to demonstrate their safety.[21][22] Millions of mammals, especially mice and rats, are used in experiments each year.[23] Knockout mice are used to help discover the functions of genes.[24][25]