In humans, proto-oncogenes can be transformed into oncogenes in three ways, all of which result in a loss of or reduction in cell regulation. An alteration of a single nucleotide base pair , called a point mutation , can arise spontaneously or as a result of environmental influences such as chemical carcinogens or ultraviolet radiation . This seemingly minor event can lead to the production of an altered protein that cannot be properly regulated. Point mutations are responsible for converting certain RAS proto-oncogenes to oncogenes. A second method of oncogenesis occurs by the process of translocation , in which a segment of the chromosome breaks off and attaches to another chromosome. If the dislocated chromosome contains a proto-oncogene, it may be removed from its regulatory controls and be continuously produced. The excess production of protein molecules disrupts the cellular process normally under their control, thereby destabilizing the delicate balance of the mechanisms of cell growth. Many leukemias and lymphomas are caused by translocations of proto-oncogenes. The third method of transformation involves an amplification in the number of copies of the proto-oncogene, which also can result in overproduction of the protein and its concomitant effects. Amplified proto-oncogenes have been found in tumours from patients with breast cancer and neuroblastoma (a tumour of the sympathetic nervous system that affects young children).