To confirm the role of cystatin C in the CD4-independent mNDK vector infection, HeLa, HeLa/CD4, TE671, TE671/CD4, 293T, and 293T/CD4 cells stably expressing cystatin C were constructed by using an MLV vector encoding cystatin C. HeLa cells expressing cystatin C were about 3 times more susceptible to the mNDK vector infection than control HeLa cells transduced by the empty MLV vector (Fig. 3A, upper panels). Here, CD4 is linked to infection.