A probe for a gene or a primer for starting amplification comprising an oligonucleotide analogue comprising two or more nucleoside units, wherein at least one of the nucleoside units is a structure of the formula (2): ##STR00001## wherein A represents a C.sub.1-C.sub.4 alkylene group, and B is an unsubstituted purin-9-yl group, an unsubstituted 2-oxo-pyrimidin-1-yl-group, a substituted purin-9-yl-group or a substituted 2-oxo-pyrimidin-1-yl group. Also a method for preventing or treating a disease preventable or treatable by the antisense or antigene activity of an oligonucleotide by administering the oligonucleotide analogue.