In mammals, microRNAs (miRNAs) regulate the transcriptional activity of about 60% of protein-encoding genes.  Individual miRNAs can each target, and repress transcription of, on average, roughly 200 messenger RNAs of protein coding genes.  The promoters of about one third of the 167 miRNAs evaluated by Vrba et al. in normal breast tissues were differentially hyper/hypo-methylated in breast cancers.  A more recent study pointed out that the 167 miRNAs evaluated by Vrba et al. were only 10% of the miRNAs found expressed in breast tissues.  This later study found that 58% of the miRNAs in breast tissue had differentially methylated regions in their promoters in breast cancers, including 278 hypermethylated miRNAs and 802 hypomethylated miRNAs.
One miRNA that is over-expressed about 100-fold in breast cancers is miR-182.  MiR-182 targets the BRCA1 messenger RNA and may be a major cause of reduced BRCA1 protein expression in many breast cancers (also see BRCA1).