cancer .
It is a sequence of DNA that has been altered or mutated from its original form, the proto-oncogene (see mutation ). Proto-oncogenes promote the specialization and division of normal cells. A change in their genetic sequence can result in uncontrolled cell growth, ultimately causing the formation of a cancerous tumour. In humans, proto-oncogenes can be transformed into oncogenes in three ways: point mutation (alteration of a single nucleotide base pair), translocation (in which a segment of the chromosome breaks off and attaches to another chromosome), or amplification (increase in the number of copies of the proto-oncogene). Oncogenes were first discovered in certain retrovirus es and were later identified as cancer-causing agents in many animals. See also {{link=Bishop, John Michael">J. Michael Bishop ; Harold Varmus .