Mononucleotide repeats are the simplest class of TRs. In eukaryotes, poly(dA:dT) tracts are ubiquitously distributed throughout the entire genome (9)(Supplementary Figure S1). Extensive studies in yeasts suggest that these non-coding repeats may perform crucial biological functions (10). Poly(dA:dT) tracts are correlated with nucleosome-depleted regions in yeasts (7,11) and in humans (12,13). Moreover, these nucleosome-depleted tracts are evolutionarily conserved among four species of yeast (14). It is hypothesized that one intrinsic property of poly(dA:dT) is to resist sharp DNA bending (15). Thus, poly(dA:dT) tracts within gene promoters can block nucleosome formation and increase transcription factor accessibility. A recent study showed that gene transcription can be fine-tuned by varying poly(dA:dT) tract length and continuity (16). However, transcriptional regulation is a dynamic and competitive process involving nucleosomes, chromatin structure and transcription factors (17).