Cat D is present outside of lysosomes. Fluorometric assay for cathepsin was performed on BE-2 neuroblastoma cell cytosolic extracts (Figure 6A) and on culture supernatants (Figure 6B) transfected with either GFP or Cat D, or non-transfected (NT). Relative fluorescence unit (RFU) per μg protein is calculated by normalizing the fluorescence signal to the amount of protein present in each sample. The experiments are representative of n = 3. Mean and standard error of the mean shown, significance is indicated by ***p < 0.001.