ESR1 and breast cancer: As CTC-ITB-01 cells do not harbor activating mutations in the ERα [22], this raises the question as to whether the therapeutic efficacy of SERDs such as fulvestrant or AZD9496 is limited due to insufficient inhibition of ERα levels and/or ERα activity, and whether treatment of ER+/HER2− breast cancer could be improved by the use of stronger ERα inhibitors.