Nuclease resistant, pyrimidine modified oligonucleotides that detect and modulate gene expression

An oligonucleotide or an oligonucleotide analog comprising at least five covalently bound nucleoside units connected together by phosphate linkages, which nucleoside units individually comprise a ribose or deoxyribose sugar portion covalently bound to a base portion, wherein a plurality of base portions are pyrimidine bases;

• and at least one of said pyrimidine bases is a modified pyrimidine base having the structure:
• ##STR2## wherein:
• X=OH or NH.sub.2 ;
• B is selected from the group consisting of:
• C-lower alkyl, N, C--CF.sub.3, C--F, C--Cl, C--Br, C--I, C-halocarbon, C--NO.sub.2, C--CF.sub.3, C--SH, C--SCH.sub.3, C--OH, C--O-lower alkyl, C--CH.sub.2 OH, C--CH.sub.2 SH, C--CH.sub.2 SCH.sub.3, C--CH.sub.2 OCH.sub.3, C--NH.sub.2, C--CH.sub.2 NH.sub.2, C--alkyl-NH.sub.2, C-benzyl, C-aryl, C-substituted aryl or benzyl wherein said substituents are selected from the group consisting of methyl, hydroxyl, alkoxy, alcohol, benzyl, phenyl, nitro, thiol, and halogen;
• provided that when B is N, then said sugar portion to which said modified pyrimidine base is bound is deoxyribose;
• and
• A is selected from the group consisting of:
• C--H, N, C--CF.sub.3, C-halocarbon, C--NO.sub.2, C--OCF.sub.3, C--SH, C--SCH.sub.3, C--OH, C--O-lower alkyl, C--CH.sub.2 OH, C--CH.sub.2 SH, C--CH.sub.2 SCH.sub.3, C--CH.sub.2 OCH.sub.3, C--NH.sub.2, C--CH.sub.2 NH.sub.2, C-benzyl, C-aryl, C-substituted aryl or benzyl wherein said substituents are selected from the group consisting of methyl, hydroxyl, alkoxy, alcohol, benzyl, phenyl, nitro, thiol, and halogen.