The retroviruses include T-cell Leukemia virus type I, HIV, and Rous Sarcoma Virus (RSV). The viral gene tax is expressed when the T-cell Leukemia virus transforms a cell altering the expression of cellular growth control genes and causing the transformed cells to become cancerous.  HIV works differently by not directly causing cells to become cancerous but by instead making those infected more susceptible to lymphoma and Kaposi's sarcoma. Many other retroviruses contain the three genes, gag, pol, and env, which do not directly cause transformation or tumor formation.