Two examples of the frequent tissue-selective motifs that match known human miRNAs. (A) The seed region of miR-203 is UGAAAUG. If we do not allow G/U pairing or left- or right-shift matches, the target motif for prefect matching is ACUUUAC, which is found only in lymph node and lung tissues. However, when the one-nucleotide shift and one G/U pairing are allowed, we find seven other motifs as their putative target motifs in 12 tissues, including the lymph node and lung. (B) miR-206a and miR-206b are two members of the miR-206 family that differ by only one base in the seed region, but their tissue-selective motifs are completely different and the target genes are also expressed in different tissues. In each example, the seed matches include (i) perfect matches and (ii) imperfect matches allowing one G/U pairing, and matches allowing a left or right shift.