Dipeptidyl peptidase IV (DPP IV, CD26, EC ) serves as a model aimed at elucidating protein sorting signals. We identify here, by MS, sev-eral tyrosine-phosphorylated proteins in a rat liver Golgi⁄endosome (G⁄E) fraction including DPP IV. We show that a pool of DPP IV is tyrosine-phosphorylated.