To confirm that costimulation was required for the antitumor effect of XmAb808s and to exclude the possibility that its binding to B7-H3 on cancer cells in the absence of T-cell engagement might enhance the efficacy of EpCAM×CD3, we designed a bispecific antibody lacking the anti-CD28–binding domain of XmAb808s (B7-H3×Null). The gene discussed is CD28; the disease is cancer.