In early female embryos, mouse and human long noncoding Xist/XIST RNA, expressed from the future inactive X chromosome (Xi), triggers X-chromosome inactivation (XCI) by associating with and spreading over the Xi, subsequently leading to gene silencing and structural conversion into heterochromatin. To investigate its association sites on Xi, we initially assumed triplex formation at Xist/XIST RNA repeat E via Hoogsteen hydrogen bonds with dsDNA, and then expanded the possible binding sites to the entire Xist/XIST RNA by defining “redundant UC-motifs”, which are clustered most densely at repeat E and scattered over the remaining regions. We targeted locked nucleic acids (LNAs) at predicted triplex forming sites, termed Junction (J)-motifs, by concatenating two types of AG-rich simple repeats (SRs), resulting in inhibition of Xist/XIST RNA localization in mouse and human cells. DNase I digestion of fixed cells combined with triplex-destabilizer distamycin treatment resulted in enhancement of Xist/XIST RNA signal intensity in RNA FISH when using probes targeting redundant UC-motifs. Here, we propose a model where Xist/XIST RNA and AG-rich SRs bring together reciprocally redundant