Several hundred oncogenes - genes which which have to potential to increase the risk of cancer - have now been identified. The way(s) in which they do so is as yet unknown.
“A fundamental aim of cancer research is to identify the molecular changes that cause normal cells to evolve into malignant tumors. Malignant transformation has been proposed to occur as a consequence of the accumulation of genomic aberrations that successively overcome the cellular barriers to malignancy. Although a concerted effort has been made to discover the complement of molecular aberrations required for malignant transformation, the spectrum of events that must occur in a single cell, including which oncogene activation and tumor suppressor gene inactivation events can cooperate to induce malignant transformation, has remained elusive.”
See: Are oncogenes sufficient to cause human cancer? in PNAS November, 2010.