The results of this screening process demonstrate that cytotoxicity changes drastically according to the Fv used and the domain order, and provide critical rules for the design of diabodies with high cytotoxicity: 1) the target on the T-lymphocyte is CD3, 2) the domain order is LH-type, 3) the target on the TFK-1 cancer cells is EGFR, 4) anti-EGFR antibody with high affinity for the epitope overlapping with the EGF-bound area on folded EGFR should be used. This evidence concerns the gene EGFR and cancer.