Stability of GP64-pseudotyped vector in human and mouse sera. GP64- or VSV-G–pseudotyped FIV vectors were incubated with either 80% competent human sera or 80% heat-inactivated human sera at 37°C for 1 hour. The vectors were similarly treated with mouse sera. After incubation, the vectors were titered on HT1080 cells. The results represent means of 3 separate titering plates and were normalized to the heat-inactivated serum controls. Data shown are representative of 2 replicate experiments. Dashed line indicates 100% of control titer.