Ubc9 binding site mutations, as well as cancer-predisposing mutation in the BRCA1 RING domain (C61G), disrupted the ability to modulate Ubc9-mediated estrogen-induced ER-α transcriptional activity in breast cancer cells [31] but did not disrupt SUMO-1 binding [29] nor auto ubiquitination activity of BRCA1 [31]. Here, ESR1 is linked to breast cancer.