CTSB and infection: When NIH3T3 cells were inoculated with the Eco-MLV vector diluted with the TE671 cell conditioned or cathepsin B-containing medium, the inhibitory effect of ConA on the Eco-MLV infection was minimized (Figs. 2A and 3A), suggesting that the Eco-MLV infection in the presence of secreted cathepsin proteases occurs at the plasma membrane.