By introducing single cysteine substitutions into various positions of the heavy and light chains of the Fab region of trastuzumab, a potent antagonist of HER2, and using thiol chemistry to link the different Fabs together, we produced a variety of monospecific F(ab′)2-like molecules with activities spanning from activation to inhibition of breast tumor cell growth. The gene discussed is ERBB2; the disease is breast neoplasm.