The rationale for designing IL-15/SuIL-15Rα-mFc with a smaller size and simpler structure is based on three considerations: (a) large proteins may have difficulties in extravasation and penetration into the tumor mass [24]; (b) the activity of IL-15 may be affected by the spatial conformation due to the potential steric hindrance between the two “arms” of Fc [25]; and (c) the binding rate of monomeric Fc to human FcRn is equivalent to that of normal Fc [26]. This evidence concerns the gene IL15 and neoplasm.