Endogenous 4ICD interacts with and potentiates ER transactivation in breast cancer cells as loss of 4ICD expression decreases ER-mediated transactivation function, whereas overexpression of 4ICD in the MCF-7 cell line increased E2 stimulation of ERE reporter activity (Zhu et al., 2006). This evidence concerns the gene ESR1 and breast carcinoma.