42. In what respect does the genome of slow-acting retroviruses differ from those of transducing viruses?
    (1) They cannot activate nearby cellular proto- oncogenes after integration into the genome of
    the host cell
    (2) They lack an oncogene
    (3) They exclude mouse mammary tumor viruses
    (4) They have acquired mutations during acquisition of an oncogene

The correct answer is:

(2) They lack an oncogene

Explanation:

• Slow-acting retroviruses differ from transducing retroviruses primarily because they lack a viral oncogene in their genome. Instead, they induce cancer by integrating near cellular proto-oncogenes and activating them (insertional mutagenesis), leading to elevated expression of the host oncogenes.​

• Transducing retroviruses carry an oncogene that has been acquired from the host genome and can directly transform cells by expressing this viral oncogene.

• Slow-acting retroviruses do activate nearby cellular proto-oncogenes after integration, so statement (1) is incorrect.

• Mouse mammary tumor viruses are examples of slow-acting retroviruses, so (3) is incorrect.

• Statement (4) about acquired mutations during oncogene acquisition is related to transducing viruses, not the defining difference.