ASS1 and B-cell chronic lymphocytic leukemia: We show that CLL cells fulfill the two key requirements, necessary for the successful depletion of arginine by inhibiting one specific transporter: (i) Primary CLL cells in general do not express ASS (Figures 1C,D) and are therefore arginine-auxotrophic and (ii) CLL cells non-redundantly rely on the import of arginine via one specific transporter, namely CAT-1 (Figure 5).