A compound having the following structure: or a stereoisomer, pharmaceutically acceptable salt or tautomer thereof, wherein: M is, at each occurrence, independently a biologically active moiety, or a fluorescent dye, provided at least one occurrence of M is not a fluorescent dye; L is a physiologically cleavable linker; L 2 and L 3 are, at each occurrence, independently an alkylene, alkenylene, alkynylene, heteroalkylene, heteroalkenylene, heteroalkynylene, heteroatomic linker, or a bond; L 4 is, at each occurrence, independently an alkylene, alkenylene, alkynylene, heteroalkylene, heteroalkenylene or heteroalkynylene linker; R 1 is, at each occurrence, independently H, alkyl or alkoxy; R 2 and R 3 are each independently H, OH, SH, alkyl, alkoxy, alkylether, heteroalkyl, -OP(=R a)(R b)R c, Q, or a protected form thereof, or L'; R 4 is, at each occurrence, independently OH, SH, O -, S -, OR d or SR d; R 5 is, at each occurrence, independently oxo, thioxo or absent; R a is O or S; R b is OH, SH, O -, S -, OR d or SR d; R c is OH, SH, O -, S -, OR d, OL', SR d, alkyl, alkoxy, heteroalkyl, heteroalkoxy, alkylether, alkoxyalkylether, phosphate, thiophosphate, phosphoalkyl, thiophosphoalkyl, phosphoalkylether or thiophosphoalkylether; R d is a counter ion; Q is, at each occurrence, independently a moiety comprising a reactive group, or protected form thereof, capable of forming a covalent bond with a complementary reactive group Q' on a targeting moiety; L' is, at each occurrence, independently a linker comprising a covalent bond to Q, a targeting moiety, a linker comprising a covalent bond to a targeting moiety, a linker comprising a covalent bond to a solid support, a linker comprising a covalent bond to a solid support residue, a linker comprising a covalent bond to a nucleoside or a linker comprising a covalent bond to a further compound of structure; m is, at each occurrence, independently an integer of two or greater; and n is an integer of one or greater. M is, at each occurrence, independently a biologically active moiety, or a fluorescent dye, provided at least one occurrence of M is not a fluorescent dye; L is a physiologically cleavable linker; L 2 and L 3 are, at each occurrence, independently an alkylene, alkenylene, alkynylene, heteroalkylene, heteroalkenylene, heteroalkynylene, heteroatomic linker, or a bond; L 4 is, at each occurrence, independently an alkylene, alkenylene, alkynylene, heteroalkylene, heteroalkenylene or heteroalkynylene linker; R 1 is, at each occurrence, independently H, alkyl or alkoxy; R 2 and R 3 are each independently H, OH, SH, alkyl, alkoxy, alkylether, heteroalkyl, -OP(=R a)(R b)R c, Q, or a protected form thereof, or L'; R 4 is, at each occurrence, independently OH, SH, O -, S -, OR d or SR d; R 5 is, at each occurrence, independently oxo, thioxo or absent; R a is O or S; R b is OH, SH, O -, S -, OR d or SR d; R c is OH, SH, O -, S -, OR d, OL', SR d, alkyl, alkoxy, heteroalkyl, heteroalkoxy, alkylether, alkoxyalkylether, phosphate, thiophosphate, phosphoalkyl, thiophosphoalkyl, phosphoalkylether or thiophosphoalkylether; R d is a counter ion; Q is, at each occurrence, independently a moiety comprising a reactive group, or protected form thereof, capable of forming a covalent bond with a complementary reactive group Q' on a targeting moiety; L' is, at each occurrence, independently a linker comprising a covalent bond to Q, a targeting moiety, a linker comprising a covalent bond to a targeting moiety, a linker comprising a covalent bond to a solid support, a linker comprising a covalent bond to a solid support residue, a linker comprising a covalent bond to a nucleoside or a linker comprising a covalent bond to a further compound of structure; m is, at each occurrence, independently an integer of two or greater; and n is an integer of one or greater.