ESR1 and breast carcinoma: In breast cancer cells where both ERα and ERβ are co-expressed ERβ may reduce ERα-mediated transcription by the formation of α/β heterodimers, with weaker transcriptional activity than the α/α homodimer or by a competition between ERα and ERβ homodimers in DNA binding, where α/α homodimers stimulate transcription more potently [33, 34].