Infectivity of virus produced from normal and NPCD55 cells. (A) Virus containing supernatants from three normal and NPC2D fibroblasts were titrated onto TZM-bl indicator cells. At 48 h post-infection, the cells were harvested and luciferase assay was performed to measure virus infectivity. Luciferase activity was normalized to virus input. (B) TZM-bl cells were infected with 5 ng of purified virus released from Normal and NPCD55 cells. Infectivity was measured by luciferase activity and normalized to Normal cells, which was arbitrarily set to 1. Error bars are representative of three independent infections.