CD4 and infection: Cathepsin B activity of SAOS-2 cells was higher than that of HeLa cells (Fig. 6B), and CA-074Me treatment conferred SAOS-2 cells susceptible to the CD4-independent mNDK vector infection (Fig. 6C), suggesting that SAOS-2 cells are completely resistant to the CD4-independent infection due to the high level of cathepsin B activity.