The term somatic is often used in biology to refer to the cells of the body, in contrast to the germ-line cells which usually give rise to the eggs or sperm. In medicine, it is a term that means (more generally) relating to the body. Somatic mutations are changes to the genes which are not passed on to the offspring. In cancer genetics, somatic mutations specifically refer to mutations arising in tumours (which are not present in healthy tissue). Such mutations are often responsible for driving the growth of tumours.