C: Biological consequences of the 2 identified N-terminal mutations. Both WT and mutant N-terminal protein fragments (p.Arg156Leu and p.Glu161Lys) were expressed in HEK293 cells as demonstrated by western blotting. The GFP-tagged protein products of both were not detected in the conditioned media of mammalian cells, as compared to wild type (WT), and appear to be retained in the intracellular compartment (mean ± s.e.m.; n = 3). Ponceau redstaining is shown to demonstrate equal protein loading for conditioned media. UT - untransfected negative control; nd - not detected; *: p=0.01094 (WT vs p.Arg156Leu) and p=0.04408 (WT vs p.Glu161Lys) by 2-tailed t-test. It has to date not been possible to test cytoplasmic retention for the two C-terminal mutations due to difficulty expressing the full length or C-terminal protein fragment in mammalian cells. 