MIER3 is a nuclear transcriptional repressor that functions as a tumor suppressor in colorectal cancer but exhibits oncogenic properties in breast cancer. In colorectal cancer, MIER3 expression is significantly reduced and associated with poor prognosis 1. MIER3 overexpression inhibits colorectal cancer cell proliferation, migration, and invasion by downregulating the transcription factor Sp1 and suppressing epithelial-mesenchymal transition (EMT) 1. Conversely, in breast cancer, MIER3 is upregulated and promotes aggressive cellular behaviors 2. Here, MIER3 forms a co-repressor complex with HDAC1, HDAC2, and Snail to silence E-cadherin expression and induce EMT 2. Mechanistically, MIER3 belongs to the MIER protein family and contains an ELM2-SANT domain, though unlike MIER1 and MIER2, MIER3 does not efficiently recruit histone deacetylases in standard cell lines 3. MIER3 was identified as a candidate breast cancer susceptibility gene at the 5q11.2 locus, where reduced mammary gland expression associates with increased tumor susceptibility 4. These findings suggest MIER3 has context-dependent roles in cancer development, serving as a potential therapeutic target and prognostic marker.