[i] Enrichment of words (7-mers) in 3′ UTRs of the 2679 genes bound by Argonaute in HEK293 cells (PAR-CLIP data from Hafner et al. 2010). Expected word occurrences were estimated from nonbound 3′ UTRs having length and nucleotide composition matching the bound 3′ UTRs (reported values are mean ± SD; see Methods). Two-tailed Bonferroni corrected P-values are estimated from the Z-scores. Many URM motifs are found among the top enriched words and significantly enriched words outside top-20 include the ARE motif and 7-mer seed sites of the miRNAs expressed at highest levels in HEK293 cells (four last rows).