A compound of Formula: or a pharmaceutically acceptable salt thereof, wherein X 1 and X 3 are each independently OH, OR 1, SH, or SR 1, provided at least one of X 1 and X 3 is OR 1, SH, or SR 1; X 2 and X 4 are each independently O or S; R 4 and R 10 are each independently H, OH, or halogen; each R 1 is independently C 1 -C 6 alkyl or -L-R 2; each R 2 is independently -O(C=O)-N(R 2a) 2, -O(C=O)-NHR 2a, -O(C=O)-R 2a, or -O(C=O)-O-R 2a; each R 2a is independently C 1 -C 20 alkyl, C 2 -C 20 alkenyl, C 2 -C 20 alkynyl, -(C 1 -C 6 alkylene)-(C 3 -C 14 cycloalkyl), or C 3 -C 20 cycloalkyl, wherein each R 2a is independently optionally substituted with 1, 2, or 3 R 2b; each R 2b is independently -OH, -SH, -NH 2, =O, =NH, =S, halogen, -N 3, -CN, C 1 -C 6 alkoxy, C 1 -C 6 alkylthio, C 1 -C 6 alkylamino, or C 1 -C 6 dialkylamino; L is L 1, L 1 -O(C=O)-L 2, L 1 -(C=O)O-L 2, L 1 -O-L 2, L 1 -S n -L 2, L 1 -O(C=O)O-L 2, L 1 -O(C=O)NR 6 -L 2, L 1 -NR 6 (C=O)O-L 2, or L 1 -O(C=O)-L 2 -O-L 3; L 1 is C 1 -C 6 alkylene, C 2 -C 6 alkenylene, C 2 -C 6 alkynylene, or C 7 -C 13 alkylarylene; L 2 is C 1 -C 6 alkylene, C 2 -C 6 alkenylene, C 2 -C 6 alkynylene, C 6 -C 10 arylene, or 5- to 10-membered heteroarylene; L 3 is C 1 -C 6 alkylene, C 2 -C 6 alkenylene, or C 2 -C 6 alkynylene; R 6 is H or C 1 -C 6 alkyl; n is 0, 1, or 2; Base 1 and Base 2 are each independently wherein A, A 1, A 2, A 3 and A 4 are each independently H, OH, SH, F, Cl, Br, I, NH 2, OR 15, SR 15, NHR 15, N(R 15) 2, or R 16; each Z is independently O, S, or NR 15; each R 15 is independently H, -C(=Z 1)R 16, -C(=Z 1)OR 16, -C(=Z 1)SR 16, -C(=Z 1)N(R 16) 2, C 1 -C 6 alkyl, C 2 -C 6 alkenyl, C 2 -C 6 alkynyl, C 3 -C 7 cycloalkyl, C 2 -C 10 heterocycloalkyl, C 6 -C 10 aryl, or C 2 -C 10 heteroaryl; each Z 1 is independently O or S; and each R 16 is independently H, C 1 -C 6 alkyl, C 2 -C 6 alkenyl, C 2 -C 6 alkynyl, C 3 -C 7 cycloalkyl, C 2 -C 10 heterocycloalkyl, C 6 -C 10 aryl, or C 2 -C 10 heteroaryl. wherein X 1 and X 3 are each independently OH, OR 1, SH, or SR 1, provided at least one of X 1 and X 3 is OR 1, SH, or SR 1; X 2 and X 4 are each independently O or S; R 4 and R 10 are each independently H, OH, or halogen; each R 1 is independently C 1 -C 6 alkyl or -L-R 2; each R 2 is independently -O(C=O)-N(R 2a) 2, -O(C=O)-NHR 2a, -O(C=O)-R 2a, or -O(C=O)-O-R 2a; each R 2a is independently C 1 -C 20 alkyl, C 2 -C 20 alkenyl, C 2 -C 20 alkynyl, -(C 1 -C 6 alkylene)-(C 3 -C 14 cycloalkyl), or C 3 -C 20 cycloalkyl, wherein each R 2a is independently optionally substituted with 1, 2, or 3 R 2b; each R 2b is independently -OH, -SH, -NH 2, =O, =NH, =S, halogen, -N 3, -CN, C 1 -C 6 alkoxy, C 1 -C 6 alkylthio, C 1 -C 6 alkylamino, or C 1 -C 6 dialkylamino; L is L 1, L 1 -O(C=O)-L 2, L 1 -(C=O)O-L 2, L 1 -O-L 2, L 1 -S n -L 2, L 1 -O(C=O)O-L 2, L 1 -O(C=O)NR 6 -L 2, L 1 -NR 6 (C=O)O-L 2, or L 1 -O(C=O)-L 2 -O-L 3; L 1 is C 1 -C 6 alkylene, C 2 -C 6 alkenylene, C 2 -C 6 alkynylene, or C 7 -C 13 alkylarylene; L 2 is C 1 -C 6 alkylene, C 2 -C 6 alkenylene, C 2 -C 6 alkynylene, C 6 -C 10 arylene, or 5- to 10-membered heteroarylene; L 3 is C 1 -C 6 alkylene, C 2 -C 6 alkenylene, or C 2 -C 6 alkynylene; R 6 is H or C 1 -C 6 alkyl; n is 0, 1, or 2; Base 1 and Base 2 are each independently wherein A, A 1, A 2, A 3 and A 4 are each independently H, OH, SH, F, Cl, Br, I, NH 2, OR 15, SR 15, NHR 15, N(R 15) 2, or R 16; each Z is independently O, S, or NR 15; each R 15 is independently H, -C(=Z 1)R 16, -C(=Z 1)OR 16, -C(=Z 1)SR 16, -C(=Z 1)N(R 16) 2, C 1 -C 6 alkyl, C 2 -C 6 alkenyl, C 2 -C 6 alkynyl, C 3 -C 7 cycloalkyl, C 2 -C 10 heterocycloalkyl, C 6 -C 10 aryl, or C 2 -C 10 heteroaryl; each Z 1 is independently O or S; and each R 16 is independently H, C 1 -C 6 alkyl, C 2 -C 6 alkenyl, C 2 -C 6 alkynyl, C 3 -C 7 cycloalkyl, C 2 -C 10 heterocycloalkyl, C 6 -C 10 aryl, or C 2 -C 10 heteroaryl.