Abstract: The present invention provides modified oligonucleotides, wherein, amongst the first four bases on the 5'-end, they have the base sequence AT or TA and, bound to one of the two O.sup.- groups of the 5'-phosphate group of the first nucleotide at the 5'-end of said oligonucleotide, they contain a radical of the general formula: ##STR1## in which R.sub.4 is a spacer group and R.sub.1, R.sub.2 and R.sub.3 are hydrogen atoms or alkyl or alkoxy radicals containing up to 3 carbon atoms.These modified oligonucleotides can be used for blocking the replication and/or expression of genes which have a sequence complementary with the modified oligonucleotides.