Single nucleotide polymorphisms (SNPs) may occur at the level of miRNA biogenesis pathway genes, pri-miRNA, pre-miRNA or the mature miRNA sequence. Such SNPs may be functional in the biogenesis and action of the mature miRNA. SNPs located to predicted miRNA target sites within 3' UTR of mRNAs have the potential to affect the efficiency of miRNA binding on its target site, create or destroy binding sites. Resulting gene deregulation can involve changes in phenotype and be eventually critical not only for the susceptibility and the onset of cancer, but also for its prognosis and therapy response prediction.