A linker compound represented by the following formula, or a salt thereof: wherein: J CB is maleimide, X'-CR b R c -C(=O)-, X'-CR b R c -C(=O)-NR e -, R a -C(=O)-, R a -C(=O)-Ar-, NH 2 -NR e -, NH 2 -NR e -C(=O)-, NH 2 -NR e -Ar-, NH 2 -O-, -C≡CH, -N 3, Ar is an optionally substituted aryl or an optionally substituted heteroaryl; X' is a halogen; is an optionally substituted cycloalkyne or an optionally substituted heterocycloalkyne, R 201, R 202 and R 203 each are independently H or an optionally substituted alkyl; Z 3 is pyridyl or wherein R 204 and R 205 are each independently optionally substituted alkyl; is an optionally substituted strained cycloalkane or an optionally substituted strained heterocycloalkene, R 301 is H or optionally substituted alkyl; R a, R b, R c, and R e, for each occurrence, are independently H or an optionally substituted alkyl; Z 1 is absent, -SO 2 NR 9 -, -NR 9 SO 2 -, -C(=O)-NR 9, -NR 9 -C(=O)-, -C(=O)-O-, -O-C(=O)-, -CH 2 -O-, -O-CH 2 -, -(CH 2 CH 2 O) p - or -(OCH 2 CH 2) p' -, wherein p and p' are independently an integer from 1 to 1000; J D is maleimide, X'-CR b R c -C(=O)-, X'-CR b R c -C(=O)-NR e -, A and A' are each independently an optionally substituted alkylene, an optionally substituted alkenylene, an optionally substituted alkynylene, an optionally substituted cycloalkylene, an optionally substituted cycloalkenylene or an optionally substituted cycloalkynylene; Q is -Z 1 -P-Z 2 -; Q' is -Z 1 '-P'-Z 2 '-; one of Z 1 and Z 2 is -C(=O)-, and the other is -NR h -; one of Z 1 ' and Z 2 ' is -C(=O)-, and the other is -NR h' -; P and P' are each independently absent, an optionally substituted alkylene,-(CH 2 -CH 2 -O) j -, -(O-CH 2 -CH 2) j -, or 1-10, in which each XX is a residue of an independently selected amino acid; j is an integer between 1 and 500; k is 0 or 1; L is -(CR 5 R 6) v -, -(CR 7 R 8) q -N(R g)-(CR 9 R 10) r -, -(CR 7 R 8) q -C(R a)(R g)-(CR 9 R 10) r or-(CR 11 R 12) s -N(R g)-(CR 13 R 14) t -N(R g ')-(CR 15 R 16) u -; R g and R g ' are each independently -(CR 17 R 18) p -Z-V; p is an integer between 1 and 5; V is H, a charged substituent or an ionizable group; Z is absent, -C(=O)NR h -alkylene- or -NR h -C(=O)-alkylene-; R h and R h ', for each occurrence, are independently H or an optionally substituted alkyl; R 5 to R 18, for each occurrence, are independently H or an optionally substituted alkyl; and q, r, s, t, u and v are each independently an integer between 0 and 10. J CB is maleimide, X'-CR b R c -C(=O)-, X'-CR b R c -C(=O)-NR e -, R a -C(=O)-, R a -C(=O)-Ar-, NH 2 -NR e -, NH 2 -NR e -C(=O)-, NH 2 -NR e -Ar-, NH 2 -O-, -C≡CH, -N 3, Ar is an optionally substituted aryl or an optionally substituted heteroaryl; X' is a halogen; is an optionally substituted cycloalkyne or an optionally substituted heterocycloalkyne, R 201, R 202 and R 203 each are independently H or an optionally substituted alkyl; Z 3 is pyridyl or wherein R 204 and R 205 are each independently optionally substituted alkyl; is an optionally substituted strained cycloalkane or an optionally substituted strained heterocycloalkene, R 301 is H or optionally substituted alkyl; R a, R b, R c, and R e, for each occurrence, are independently H or an optionally substituted alkyl; Z 1 is absent, -SO 2 NR 9 -, -NR 9 SO 2 -, -C(=O)-NR 9, -NR 9 -C(=O)-, -C(=O)-O-, -O-C(=O)-, -CH 2 -O-, -O-CH 2 -, -(CH 2 CH 2 O) p - or -(OCH 2 CH 2) p' -, wherein p and p' are independently an integer from 1 to 1000; J D is maleimide, X'-CR b R c -C(=O)-, X'-CR b R c -C(=O)-NR e -, A and A' are each independently an optionally substituted alkylene, an optionally substituted alkenylene, an optionally substituted alkynylene, an optionally substituted cycloalkylene, an optionally substituted cycloalkenylene or an optionally substituted cycloalkynylene; Q is -Z 1 -P-Z 2 -; Q' is -Z 1 '-P'-Z 2 '-; one of Z 1 and Z 2 is -C(=O)-, and the other is -NR h -; one of Z 1 ' and Z 2 ' is -C(=O)-, and the other is -NR h' -; P and P' are each independently absent, an optionally substituted alkylene,-(CH 2 -CH 2 -O) j -, -(O-CH 2 -CH 2) j -, or 1-10, in which each XX is a residue of an independently selected amino acid; j is an integer between 1 and 500; k is 0 or 1; L is -(CR 5 R 6) v -, -(CR 7 R 8) q -N(R g)-(CR 9 R 10) r -, -(CR 7 R 8) q -C(R a)(R g)-(CR 9 R 10) r or-(CR 11 R 12) s -N(R g)-(CR 13 R 14) t -N(R g ')-(CR 15 R 16) u -; R g and R g ' are each independently -(CR 17 R 18) p -Z-V; p is an integer between 1 and 5; V is H, a charged substituent or an ionizable group; Z is absent, -C(=O)NR h -alkylene- or -NR h -C(=O)-alkylene-; R h and R h ', for each occurrence, are independently H or an optionally substituted alkyl; R 5 to R 18, for each occurrence, are independently H or an optionally substituted alkyl; and q, r, s, t, u and v are each independently an integer between 0 and 10.