Oncogenes are genes that are critical to the cause and proliferation of cancer. Normal, non-mutated forms of these genes (proto-oncogenes) code for proteins that regulate cell growth and differentiation. In contrast to tumor suppressors, they are involved in cancer through a gain of function, not loss of function (caused by a mutation, increased expression, or chromosome rearrangement). Many current cancer therapies focus on targeting proteins that are encoded by oncogenes.
There are many known and proposed mechanisms of oncogenesis, which can lead to abnormal cell growth. Some well-studied drivers of malignancy include mutation of proteins involved in intracellular signaling (such as MAPK/ERK and PI3K/AKT/MTOR pathways), mutation of growth factor receptors (ex. PDGF), and excess expression of growth factors (ex. ErbB2/HER2).