The mesoderm induction early response (MIER) protein family (also known as the fibroblast growth factor (FGF)-regulated immediate-early protein family) comprises a group of proteins that are activated by FGF (fibroblast growth factor). This suggests that MEIR proteins may play a significant role in FGF-regulated cellular activities and in the progression of certain cancers. MIER proteins contain one SANT domain, which is involved in transcriptional activation and repression, and one ELM2 domain, which was first characterized in egl-27, a gene that is critically involved in embryonic patterning of C. elegans. MIER1, formerly known as early response 1 (ER1), was first cloned and characterized in Xenopus. Expression of MIER1 is negligible in most normal tissues, but has been found to be upregulated in breast carcinoma cell lines and tumors. MIER1 functions as a transcriptional repressor of a number of genes including Sp1 target genes, most likely through interaction with HDAC1.