Despite insufficient knowledge of the functions of Fc receptors because of the diverse profiles of the receptors on discrete leukocytes, Fc engineering works for (i) improving effector functions by selective FcγR affinity in tumor targeting purposes, (ii) maximizing serum half-life by pH-selective FcRn affinity for various disease categories such as cancer, infectious diseases or autoimmune therapy, (iii) molecular downsizing to half relative to that of native antibodies by blocking homodimerization, and (iv) conferring bispecificity by maximizing the efficiency of Fc heterodimerization. The gene discussed is FCGRT; the disease is neoplasm.