Cathepsin D trafficking is normal in the absence of Surf4. (A) Genotyping of Surf4 KO cells identified frameshift causing mutations (1-bp insertion and 2-bp deletion). (B) Confocal immunofluorescence images of the localization of cathepsin D and LAMP1 in control and Surf4 KO cells. Scale bar, 10 µm; inset, 3.38 µm wide. (C) Immunoblot of cathepsin D protein levels in control and Surf4 KO cells. Vinculin was used as a loading control. (D) Quantification of cathepsin D levels from four independent experiments (mean ± SEM; unpaired t test; **, P < 0.01).