Using new molecular technologies, scientists can genetically engineer mouse embryos from scratch to contain precisely the same biological defects that cause diabetes, cancer, multiple sclerosis, cystic fibrosis, arthritis and many other human ailments. The new breeds, which have never existed in nature, can be dissected and analyzed by the hundreds or thousands until the biological mechanism underlying their problem is revealed. The use of animals in research is already fraught with ethical dilemmas, and many see this latest development as the dawn of a dark era in medical research.