The only oncogenic virus whose pathogenicity has been proved without doubt is:

Ans. is 'a' i.e., HTLV - IHTLV-1 has been established as the causative agent of adult T-cell leukemia-lymphomas (ATL) as well as a nervous system degenerative disorder called tropical spastic paraparesis. It does not carry an oncogene. The human lymphotropic viruses have a marked affinity for mature T cells. HTLV - 1 is expressed at very low levels in infected individuals. It appears that the viral promoter enhancer sequences in the long terminal repeat may be responsive to signals associated with the activation and proliferation of T cells. If so, the replication of the viruses may be linked to the replication of the host cells - a strategy that would ensure efficient propagation of the virus