Tumours are abnormal cell growths resulting from uncontrolled cell division and can occur in any tissue or organ

• Diseases caused by the growth of tumours are collectively known as cancers
  

Tumours are caused by mutations to the genes that control the cell cycle 

• Mutations may be caused by proofreading errors that may occur during DNA replication (in the S phase of interphase)
  

• Mutations may also be caused by external agents called mutagens (a cancer-causing mutagen is called a carcinogen)
  

  • Physical mutagens include sources of radiation – such as X-rays and ultraviolet (UV) light 

  • Chemical mutagens may include reactive oxygen species and certain metals (e.g. arsenic)

  • Biological agents include certain viruses, bacteria or mobile genetic elements (transposons)

Most cancers are caused by mutations to two basic classes of genes – proto-oncogenes and tumour suppressor genes

• Proto-oncogenes code for proteins that stimulate the cell cycle and promote cell growth and proliferation
  

• Tumour suppressor genes code for proteins that repress cell cycle progression and promote apoptosis
  

When a proto-oncogene is mutated or subjected to increased expression it becomes an oncogene (cancer-causing gene)

• Tumour suppressor genes are sometimes referred to as anti-oncogenes, as their normal function prevents cancer