Cancer ;  Cell  division , like  all other  biological  processes,  is under  genetic control. Certain  genes must regulate the processes  of cell growth  and division  in response to intracellular, intercellular and environmental single . Modern scientists  have  identified and characterized about  twenty allelic  pairs  of such genes in the genomes of different vertebrates, including humans . These genes are  called  proto- oncogenes, If    during  DNA replication, a proto oncogene happens to  change (= mutate), the affected cells is  likely to exhibit uncontrolled  growth  and division  ( neoplastic  cell) , Further  mutations  in such  cells  usually cause cancer.

A mutated proto oncogen is  called  oncogene. In  tissues other  than haemopoetic  and  lymphoid tissues , the    uncontrolled growth  and proliferation of an oncogene containing  cell results into the formation  of a cell cass called tumor, such  a tumor may   remain   harmless ( bengn tumor )  for indefinite  time  and , if it caused some  physical  discomfort it can be  removed  surgically. If  on the other hand, further oncogenic  mutations occur in certain  cells of such  a tumor , the  becomes   then  the progeny of mutant cells  start   breaking loose  from the  tumor and invading  through blood  circulation, other  tissues and forming    secondary  tumors ( a process called  metastasis )  in different  tissues. This  is the  disease  of Cancer.

Cancerous tumors  arising from epithelial  cells  carcinomas, and  those arising  from connective tissues  or muscle cells  are called  sarcomas. Cancerous  mutation in  myelogenous cells  of bone  marrow , or  lymphogenous   cells  of lymploid tissues result  into  uncontrolled  production  of white  blood corpuscles, This  is called  blood  cancer  or leukemia.

Cancer, sooner  or later  kill the patients because  cancerous cells  complete  with  and deprive normal tissues of essential nutrients , so that  normal tissues  gradually meet a  starvation  death.

Probability  of cancer generating  oncogenic  mutations  have  presently  increased  manyfold  due  to the  effects of  a number of environmental  mutagenic  agents. The  latter include ionizing  radiations  (e.g. radiations from  Hiroshima and  Nagasaki  bombings )  pollutant   chemicals  ( carcinogens )  of which  cigarette smoke  is said  to be most  potent  carcinogen,  certain  physical  irritants , and  hereditary  tendencies, That  is why  cancer is progressively becoming  one of the most  potent fatal diseases of man ,It is feared  that  roughly one person  in five  specially in  prosperous countries, would die of cancer in the future.