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 1 is, at each occurrence, independently an alkylene, alkenylene or alkynylene linker; L 2 and L 3 are, at each occurrence, independently an optional alkylene, alkenylene or alkynylene 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, alkylaminyl, alkylcarbonyl, alkoxycarbonyl, Q or a protected form thereof, or L', wherein the alkyl, alkoxy, alkylether, heteroalkyl, alkylaminyl, alkylcarbonyl and alkyloxycarbonyl are optionally substituted with hydroxyl, amino, sulfhydryl, phosphate, thiophosphate, phosphoalkyl, thiophosphoalkyl, phosphoalkylether or thiophosphoalkylether, or combinations thereof; R 4 is, at each occurrence, independently OH, O - or OR d; R 5 is, at each occurrence, oxo; R d is a cation; 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 solid support or solid support residue, a linker comprising a covalent bond to a solid support or solid support residue or a linker comprising a covalent bond to a further compound of structure; m is, at each occurrence, independently an integer of zero or greater, provided that at least one occurrence of m is an integer of three 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 1 is, at each occurrence, independently an alkylene, alkenylene or alkynylene linker; L 2 and L 3 are, at each occurrence, independently an optional alkylene, alkenylene or alkynylene 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, alkylaminyl, alkylcarbonyl, alkoxycarbonyl, Q or a protected form thereof, or L', wherein the alkyl, alkoxy, alkylether, heteroalkyl, alkylaminyl, alkylcarbonyl and alkyloxycarbonyl are optionally substituted with hydroxyl, amino, sulfhydryl, phosphate, thiophosphate, phosphoalkyl, thiophosphoalkyl, phosphoalkylether or thiophosphoalkylether, or combinations thereof; R 4 is, at each occurrence, independently OH, O - or OR d; R 5 is, at each occurrence, oxo; R d is a cation; 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 solid support or solid support residue, a linker comprising a covalent bond to a solid support or solid support residue or a linker comprising a covalent bond to a further compound of structure; m is, at each occurrence, independently an integer of zero or greater, provided that at least one occurrence of m is an integer of three or greater; and n is an integer of one or greater.