A 5' modified nucleoside having Formula I: wherein: Bx is a heterocyclic base moiety; T 1 is a protected phosphorus moiety; M 1 is H, hydroxyl or OR 1; M 2 is hydroxyl, OR 1 orN(R 1)(R 2); each R 1 and R 2 is, independently, C 1 -C 6 alkyl or substituted C 1 -C 6 alkyl; r is 0 or 1; three of q 1, q 2, q 3 and q 4 are each H and either: i) one of q 1, q 2, q 3 and q 4 is hydroxyl, substituted oxy, O-C 1 -C 6 alkyl, substituted O-C 1 -C 6 alkyl, S-C 1 -C 6 alkyl, substituted S-C 1 -C 6 alkyl, N(R 3)-C 1 -C 6 alkyl or substituted N(R 3)-C 1 -C 6 alkyl, or alternatively ii) one of q 1 and q 2 is F, O-P(=O), O-C 1 -C 6 alkyl or substituted O-C 1 -C 6 alkyl wherein each substituent group is, independently, C(=O)OCH 3, C(=O)N (CH 3), C(=O)N -(CH 2) 2 N(CH 3) 2, a heterocycle or an optionally protected hydroxyl group, or alternatively iii) one of q 3 and q 4 is F or O-R 8 wherein R 8 is a protecting group or a substituted O-C 1 -C 6 alkyl wherein each substituent group is, independently, one or more optionally protected hydroxyl groups; R 3 is H, C 1 -C 6 alkyl or substituted C 1 -C 6 alkyl; G is F, OCH 3, O(CH 2) 2 -OCH 3, OCH 2 C(=O)-N CH 3 or OCH 2 C(=O)-N -(CH 2) 2 -N(CH 3) 2; g is 1; each substituted group comprises one or more optionally protected substituent groups independently selected from H, halogen, OJ 1, N(J 1)(J 2), =NJ 1, SJ 1, N 3, CN, OC(=L)J 1, OC(=L)N(J 1)(J 2), C(=L)N(J 1)(J 2), C(=L)N -(CH 2) 2 N(J 1)(J 2), mono or poly cyclic ring system, a phosphate group or a phosphorus moiety; L is O, S or NJ 3; and each J 1, J 2 and J 3 is, independently, H, or C 1 -C 6 alkyl. Bx is a heterocyclic base moiety; T 1 is a protected phosphorus moiety; M 1 is H, hydroxyl or OR 1; M 2 is hydroxyl, OR 1 orN(R 1)(R 2); each R 1 and R 2 is, independently, C 1 -C 6 alkyl or substituted C 1 -C 6 alkyl; r is 0 or 1; three of q 1, q 2, q 3 and q 4 are each H and either: i) one of q 1, q 2, q 3 and q 4 is hydroxyl, substituted oxy, O-C 1 -C 6 alkyl, substituted O-C 1 -C 6 alkyl, S-C 1 -C 6 alkyl, substituted S-C 1 -C 6 alkyl, N(R 3)-C 1 -C 6 alkyl or substituted N(R 3)-C 1 -C 6 alkyl, or alternatively ii) one of q 1 and q 2 is F, O-P(=O), O-C 1 -C 6 alkyl or substituted O-C 1 -C 6 alkyl wherein each substituent group is, independently, C(=O)OCH 3, C(=O)N (CH 3), C(=O)N -(CH 2) 2 N(CH 3) 2, a heterocycle or an optionally protected hydroxyl group, or alternatively iii) one of q 3 and q 4 is F or O-R 8 wherein R 8 is a protecting group or a substituted O-C 1 -C 6 alkyl wherein each substituent group is, independently, one or more optionally protected hydroxyl groups; i) one of q 1, q 2, q 3 and q 4 is hydroxyl, substituted oxy, O-C 1 -C 6 alkyl, substituted O-C 1 -C 6 alkyl, S-C 1 -C 6 alkyl, substituted S-C 1 -C 6 alkyl, N(R 3)-C 1 -C 6 alkyl or substituted N(R 3)-C 1 -C 6 alkyl, or alternatively ii) one of q 1 and q 2 is F, O-P(=O), O-C 1 -C 6 alkyl or substituted O-C 1 -C 6 alkyl wherein each substituent group is, independently, C(=O)OCH 3, C(=O)N (CH 3), C(=O)N -(CH 2) 2 N(CH 3) 2, a heterocycle or an optionally protected hydroxyl group, or alternatively iii) one of q 3 and q 4 is F or O-R 8 wherein R 8 is a protecting group or a substituted O-C 1 -C 6 alkyl wherein each substituent group is, independently, one or more optionally protected hydroxyl groups; R 3 is H, C 1 -C 6 alkyl or substituted C 1 -C 6 alkyl; G is F, OCH 3, O(CH 2) 2 -OCH 3, OCH 2 C(=O)-N CH 3 or OCH 2 C(=O)-N -(CH 2) 2 -N(CH 3) 2; g is 1; each substituted group comprises one or more optionally protected substituent groups independently selected from H, halogen, OJ 1, N(J 1)(J 2), =NJ 1, SJ 1, N 3, CN, OC(=L)J 1, OC(=L)N(J 1)(J 2), C(=L)N(J 1)(J 2), C(=L)N -(CH 2) 2 N(J 1)(J 2), mono or poly cyclic ring system, a phosphate group or a phosphorus moiety; L is O, S or NJ 3; and each J 1, J 2 and J 3 is, independently, H, or C 1 -C 6 alkyl.