The theoretical risk from oncogene encoded proteins is limited to growth factors, which may be secreted by cells in which biologically derived products might be manufactured. Because these peptides do not replicate, their effect is finite. In addition, their effects are reversible. Growth factors do not ordinarily appear to be oncogenic. Even under circumstances in which they contribute to oncogenicity, repeated administration of high concentrations of growth factors (several micrograms per kilogram) would appear to be required for them to serve as co-factors in the carcinogenic process, and the resulting tumors appear to remain dependent upon the continued presence of the growth factor for continued tumor growth.