Additionally, the infectivity of the WT and mutant Env-carrying MMTV virions was blocked by heat-inactivation (60°C, 10 minutes) and an inhibitor of reverse transcription, 3′-azido-3′-deoxythymidine (AZT; 37 μM) (Figure 4C), ruling out a non-specific mechanism of vector transfer/showing that vector transfer is due to an authentic, reverse transcriptase-dependent infection process. Here, ERVW-1 is linked to infection.