Transformation of cells means making cells immortal.  The cells pile up and don’t have spaces between them.  Transformed cells have been known about since the middle of the twentieth century.  One immortal cell line used in laboratories are HeLa cells.  Transformed cells have loss of anchorage dependence and  contact inhibition.  On the other hand, oncogenesis is the  development of cancer.  A tumor forms and is an enlargement caused by abnormal tissue growth. The tumor may be benign or malignant.  Transformation is changing the properties of cells in culture; and the cells become immortal.  Oncogenesis is the development of a tumor which requires additional genetic changes.

Viruses can transform cells; and this gives them oncogenic potential.  The cancer arises because transformed cells keep dividing and undergo mutations that make them a cancer, due to repeated genome replications which gives them a higher number of chances to mutate.    Cancer can also be caused by a viral infection.  RNA and DNA viruses have been shown to transform cells. These cells  may go on to form cancer in humans.  Rous Sarcoma Virus (RSV) has been shown to form tumors.  It does this by acquiring a src oncogene.  Other oncogenes are involved in causing other types of cancers.  The oncogenes become activated by loss of their genetic control mechanisms; and thus become tumorigenic.  There exist approximately sixty of these oncogenes.  All these genes are involved in cell growth; whose control is ultimately lost.