Retroviruses with two oncogenes.

Less than a decade ago the first retroviral oncogene was discovered [1] and shown to derive from a normal cellular gene [2]. To date over 20 different retroviral oncogenes have been found, all derived from cellular counterparts. How a single oncogene can cause cancer is still not fully understood. Natural cancers are known to involve multistep molecular changes, raising a paradox, as compared to cancers induced by retroviral oncogenes. To turn a cellular gene into an active retroviral oncogene, an overexpression due to the very potent retroviral promoter may suffice in a few cases, whereas in most cases several modifications are probably required. These modifications might involve enhanced transcription, truncation of