Clearly, this retroviral gene fills an important niche in mammalian biology, but, on the other hand, mammals were able to reproduce perfectly well before they were first infected.

• 显然，这种逆转录病毒基因填补了哺乳动物生物学的一个小生境，但是从另一方面来说，在第一次感染此类病毒以前，哺乳动物已经能够完美的生殖了。