A compound of formula or a salt thereof: wherein Y 1 is CR 6 or N, Y 2 is CR 7 or N, Y 3 is CR 8 or N, R 6, R 7 and R 8 are each independently, a hydrogen atom, halogen, cyano, substituted or unsubstituted alkyl, substituted or unsubstituted alkenyl, substituted or unsubstituted alkynyl, substituted or unsubstituted amino, substituted or unsubstituted alkoxy, substituted or unsubstituted alkylcarbonylamino, substituted or unsubstituted alkenylcarbonylamino, substituted or unsubstituted alkynylcarbonylamino, substituted or unsubstituted alkylcarbamoyl, substituted or unsubstituted alkenylcarbamoyl, or substituted or unsubstituted alkynylcarbamoyl, Bx is a nucleic acid base moiety, Z 1 and Z 2 are each independently, a hydrogen atom, a hydroxyl protecting group selected from substituted or unsubstituted alkyl, substituted or unsubstituted alkenyl, formyl, aliphatic acyl, aromatic acyl, tetrahydropyranyl, tetrahydrothiopyranyl, tetrahydrofuranyl, tetrahydrothiofuranyl, silyl, alkyloxymethyl, alkyloxylated alkyloxymethyl, halogeno alkyloxymethyl, alkyloxylated ethyl, halogenated ethyl, methyl substituted by 1 to 3 aryl, methyl substituted by 1 to 3 aryl with the aryl ring being substituted by alkyl, alkyloxy, halogen or cyano, alkyloxycarbonyl, aryl substituted by halogen, alkyloxy or nitro, alkyloxycarbonyl substituted by halogen or trialkylsilyl, alkenyloxycarbonyl, or aralkyloxycarbonyl having an aryl ring optionally substituted by one or two alkyloxy or nitro, or from 1-(2-chloroethoxy)ethyl, 2-trimethylsilylethyl, trimethylsilyl, triethylsilyl, t-butyldimethylsilyl, t-butyldiphenylsilyl, triphenylsilyl, triisopropylsilyl, benzoyl formate, chloroacetyl, trichloroacetyl, trifluoroacetyl, pivaloyl, isobutyryl, methansulfonyl, p-toluenesulfonyl, trifluoromethanesulfonyl, 9-phenylxanthin-9-yl or 9-(p-methoxyphenyl)xanthin-9-yl; or a reactive phosphorus group wherein the reactive phosphorous group is selected from a group of the formula: -P(OC 2 H 4 CN) (N(i-Pr)2) or a group of the formula: -P(=O)H, R 1 and R 2 are each independently, a hydrogen atom, halogen, cyano, substituted or unsubstituted alkyl, substituted or unsubstituted alkenyl, or substituted or unsubstituted alkynyl, R 3 are each independently, a hydrogen atom, halogen, cyano, substituted or unsubstituted alkyl, substituted or unsubstituted alkenyl, or substituted or unsubstituted alkynyl, R 4 are each independently, a hydrogen atom, halogen, cyano, substituted or unsubstituted alkyl, substituted or unsubstituted alkenyl, or substituted or unsubstituted alkynyl, R 5 is a hydrogen atom, halogen, cyano, substituted or unsubstituted alkyl, substituted or unsubstituted alkenyl, or substituted or unsubstituted alkynyl, and n is an integer of 0 to 3. Y 1 is CR 6 or N, Y 2 is CR 7 or N, Y 3 is CR 8 or N, R 6, R 7 and R 8 are each independently, a hydrogen atom, halogen, cyano, substituted or unsubstituted alkyl, substituted or unsubstituted alkenyl, substituted or unsubstituted alkynyl, substituted or unsubstituted amino, substituted or unsubstituted alkoxy, substituted or unsubstituted alkylcarbonylamino, substituted or unsubstituted alkenylcarbonylamino, substituted or unsubstituted alkynylcarbonylamino, substituted or unsubstituted alkylcarbamoyl, substituted or unsubstituted alkenylcarbamoyl, or substituted or unsubstituted alkynylcarbamoyl, Bx is a nucleic acid base moiety, Z 1 and Z 2 are each independently, a hydrogen atom, a hydroxyl protecting group selected from substituted or unsubstituted alkyl, substituted or unsubstituted alkenyl, formyl, aliphatic acyl, aromatic acyl, tetrahydropyranyl, tetrahydrothiopyranyl, tetrahydrofuranyl, tetrahydrothiofuranyl, silyl, alkyloxymethyl, alkyloxylated alkyloxymethyl, halogeno alkyloxymethyl, alkyloxylated ethyl, halogenated ethyl, methyl substituted by 1 to 3 aryl, methyl substituted by 1 to 3 aryl with the aryl ring being substituted by alkyl, alkyloxy, halogen or cyano, alkyloxycarbonyl, aryl substituted by halogen, alkyloxy or nitro, alkyloxycarbonyl substituted by halogen or trialkylsilyl, alkenyloxycarbonyl, or aralkyloxycarbonyl having an aryl ring optionally substituted by one or two alkyloxy or nitro, or from 1-(2-chloroethoxy)ethyl, 2-trimethylsilylethyl, trimethylsilyl, triethylsilyl, t-butyldimethylsilyl, t-butyldiphenylsilyl, triphenylsilyl, triisopropylsilyl, benzoyl formate, chloroacetyl, trichloroacetyl, trifluoroacetyl, pivaloyl, isobutyryl, methansulfonyl, p-toluenesulfonyl, trifluoromethanesulfonyl, 9-phenylxanthin-9-yl or 9-(p-methoxyphenyl)xanthin-9-yl; or a reactive phosphorus group wherein the reactive phosphorous group is selected from a group of the formula: -P(OC 2 H 4 CN) (N(i-Pr)2) or a group of the formula: -P(=O)H, R 1 and R 2 are each independently, a hydrogen atom, halogen, cyano, substituted or unsubstituted alkyl, substituted or unsubstituted alkenyl, or substituted or unsubstituted alkynyl, R 3 are each independently, a hydrogen atom, halogen, cyano, substituted or unsubstituted alkyl, substituted or unsubstituted alkenyl, or substituted or unsubstituted alkynyl, R 4 are each independently, a hydrogen atom, halogen, cyano, substituted or unsubstituted alkyl, substituted or unsubstituted alkenyl, or substituted or unsubstituted alkynyl, R 5 is a hydrogen atom, halogen, cyano, substituted or unsubstituted alkyl, substituted or unsubstituted alkenyl, or substituted or unsubstituted alkynyl, and n is an integer of 0 to 3.