SNPs and INDELs in 3′-UTRs of all RefSeq genes were collected using the ALL SNPs 137 (dbSNP build 137) track in the UCSC table browser (34). Specifically, we selected the following filter options: 3′-UTRs, SNP, insertion, deletion and INDEL for the mouse (mm10) and human (hg19) genomes. Mature miRNA sequences were downloaded from miRBase (release 20) and 3′-UTR sequences were downloaded from UCSC table browser. Perl codes for target prediction and context + score were downloaded from TargetScan (release 6.2) (31). We used the TargetScan context + score to assess the impact of polymorphism on miRNA–mRNA interaction. For each SNP and INDEL in the 3′-UTR of RefSeq genes, ancestral alleles were determined using pairwise sequence alignment data (hg19 and pantor2 for human, mm10 and rn4 for mouse) from the UCSC genome browser. Target site conservation was determined using the multiple sequence alignments data downloaded from Targetscan 6.2. The polymorphic miRNA target sites were assigned into four classes: ‘D' (the derived allele disrupts a conserved miRNA site), ‘N' (the derived allele disrupts a nonconserved miRNA site), ‘C' (the derived allele creates
