The mesoderm induction early response (MIER) protein family is known as fibroblast growth factor (FGF)-regulated immediate-early protein family. Activated by FGF, MIER proteins may have a pivotal role in FGF-regulated cellular activities, indicating a potential influence in the progression of certain cancers [PMID:15117948]. MIER3 localizes to the nucleus and primarily functions as a transcriptional repressor. Rat mammary carcinoma susceptibility 1b (Mcs1b) is a quantitative trait locus on RN02 that confers decreased susceptibility when Copenhagen (COP)-resistant alleles are introgressed into a Wistar Furth (WF)-susceptible genome. MIER3 is a candidate Mcs1b gene [PMID:22993404].