Because the CD13 inhibitor bestatin is an approved drug for adjuvant treatment of cancer, our finding that inhibition of CD13 activity with small molecules and monoclonal antibodies significantly facilitated tumor cell susceptibility to TRAIL-mediated killing, both in vitro and in vivo, may feasibly provide an effective way to enhance TRAIL- or DR4 activation-induced death of tumor cells, even in clinical settings. Here, ANPEP is linked to cancer.