To explore the role of mononucleotide repeats in humans, we performed a computational analysis by integrating data from a number of relevant databases, including whole-genome sequences (27), Gene Expression Omnibus (GEO) data sets (28) and Agobinding sites (29). Six model organisms were used in our analysis: Saccharomyces cerevisiae, Caenorhabditis elegans, Drosophila melanogaster, Rattus norvegicus, Mus musculus and Homo sapiens (Supplementary Table S1). The distribution frequency of mononucleotide repeats has previously been investigated by counting poly(dA:dT) tracts in double-stranded DNA (9). However, this previously used counting method might not be well suited to uncovering biological functions because it does not reflect the imbalance of A- and T-repeats between the two DNA strands. In this study, poly(dA:dT) tracts were counted separately as sense A- and sense T-repeats relative to the TSS.