Transgenic mouse, conceptual computer artwork. The mouse is standing next to a model of a strand of DNA (deoxyribonucleic acid), the genetic material that controls the growth and development of all living organisms. The horizontal bars are the nucleotides bases that join the two spiralling sugar phosphate strands. It is the order along the strands of these nucleotides bases, of which there are four types, that is the genetic code. Mice are widely used as laboratory animals because of their close genetic and physiological similarities to humans.