Results from experiments conducted by Soto-Reyes et al.[34] indicate a negative effect of methylation on the function and expression of miR-125b1. Therefore, they concluded that DNA methylation has a part in silencing the gene. Furthermore, some miRNA's are epigenetically silenced early on in breast cancer, and therefore these miRNA's could potentially be useful as tumor markers.[34] The epigenetic silencing of miRNA genes by aberrant DNA methylation is a frequent event in cancer cells; almost one third of miRNA promoters active in normal mammary cells were found hypermethylated in breast cancer cells - that is a several fold greater proportion than is usually observed for protein coding genes.[35]