Genomic locations of mature miRNAs were downloaded from miRBase (38). For each miRNA, we collected all SNPs and INDELs in the seed regions from dbSNP (39). We found 271 SNPs and 23 INDELs in human miRNA seeds, and 144 SNPs and 36 INDELs in mouse miRNA seeds. The mutant allele of each polymorphism disrupts the binding of the miRNA to its original target sites and may create new target sites. For each polymorphism in miRNA seeds and for each miRNA target, we computed the difference between context + scores for the mutant allele and reference allele.
