This is to provide a single-stranded oligonucleotide that can control a targetgenewith high efficiency and can be easily produced.It is a single-stranded oligonucleotide represented by the formula (I),wherein X isrepresented by Xa-Xb, Xa is coupled with Y, and Xb and Y hybridize. Xa iscomposed of 1to 40 nucleotides, and contains at least one modified-nucleotide. Xb iscomposed of 4 to 40nucleotides, and contains at least one modified-nucleotide. Y is composed of 4to 40nucleotides, and contains at least one ribonucleotide. Xz and Yz are composedof 5 to 40nucleotides, and contain at least one modified-nucleotide. Nucleotidesequences X, Xz andYz have an antisense sequence capable of hybridizing with a target RNA. Lx andLy arecomposed of 0 to 20 nucleotides.[X z ¨L x]m - X - Y - [L y -Y z ]n ( I )
