Abnormal growth of neoplastic cells can arise secondary to mutation of either proto-oncogenes or anti oncogenes. Proto-oncogenes stimulate cell proliferation. Overexpression or amplification of a proto-oncogene leads to increased cellular proliferation and neoplastic growth. Anti-oncogenes, in contrast, are tumor suppressors in that they inhibit cellular proliferation. Inactivation of anti-oncogenes contributes to tumor development.