Compared to other myeloma antigenic peptide epitopes that have been published, such as the anti-FcRH5/CD3, in which the peptide-induced CTLs had 85% killing efficiency in vitro against target cell lines at an E:T ratio of 10:1 (24), and two other weaker ones, XBP1 (25) and CS1 (26), which induced a CTL killing efficiency of about 50% against myeloma cell lines at an E:T ratio of 10:1, the APAK4 antigenic peptide-induced CTLs in our study had lower killing efficiency (27%–60%). Here, XBP1 is linked to plasma cell myeloma.