The IFN-induced block occurs early in the MLV replication cycle. OMK cells were treated or not with 1000 U/ml of IFN-β and challenged 24 h later with DNase-treated GFP-expressing SIVmac (SIV), HIV-1 (HIV), N-, B-, or NB-tropic MLV vectors. The same experiment was performed in parallel in the presence of AZT at 5 μM during infections. Total DNA was extracted 6 h post-transduction and the amount of reverse transcripts was estimated by PCR using GFP primers. A PCR on CypA was also performed as a control.