Potential enhancement or inhibition resulting from DPP4 interactions with regulatory proteins. (A) Molecules that are stimulatory in the full-length form may lose activity after DPP4 truncation and act as competitive inhibitors by diminishing the function of the full-length protein through binding to the cognate receptor with higher affinity than that of the full-length molecule. (B) Molecules that are inhibitory in the full-length form may lose their suppressive ability when truncated by DPP4 and by binding to the cognate receptor with higher affinity, allowing for blockage of the full-length molecule’s suppressive activity. (C) Stimulatory or inhibitory molecules may increase their efficacy when truncated.