Mutant antibodies and conjugation thereof

The present invention relates to a polypeptide comprising 7 β-strands A, B, C, D, E, F, and G sequentially connected together by connecting chains of amino acids, and a first α-helix sequentially located on the EF chain between β-strands E and F, wherein the β-strands are arranged so as to form a first β-sheet comprising β-strands A, B, D, and E, and a second β-sheet comprising β-strands C, F and G, said first and second β-sheets being covalently bonded together so as to form a first Ig domain; wherein the EF chain between β-strands E and F comprises the sequence X1-X2-X3-X4-K5H6 (SEQ ID NO:98), and X1, X3 and X4 are each independently any amino acid residue, characterized in that X2 is selected from the group consisting of A, G, I, V, L, R, S, T, Q, P, N, M, H, W, and pharmaceutically acceptable salts, stereoisomers, tautomers, solvates, and prodrugs thereof.