HTT and juvenile Huntington disease: To address this, we performed detailed characterization of two well-known triplet repeat loci: the CGG repeat in the 5' untranslated region (UTR) of FMR1, associated with Fragile X syndrome (FXS) (Fig. 3a,b), and the CAG repeat in the coding region of HTT, associated with Huntington’s disease (Fig. 3d,e).