It has been shown that reducing the antibody’s size exponentiallyincreases diffusion through normal and tumor tissues; a 2-fold decreasecan yield a 4-fold increase in tissue penetration., Previously,we developed a VH-Fc fusion protein, 2A10-VH-Fc, approximately halfthe size of a normal IgG. After radiolabelingwith zirconium-89 (89Zr), its efficacy was assessed inhuman colorectal tumor models by positron emission tomography (PET)imaging against m912, an anti-MSLN IgG1. The gene discussed is MSLN; the disease is neoplasm.