To determine whether the tumor-suppressive effects of CDK8 inhibition and potentiation of fulvestrant activity observed in vitro would be recapitulated in vivo, we treated mice bearing MCF7 xenograft tumors with a vehicle, Senexin B or fulvestrant alone or with a combination of Senexin B and fulvestrant over a period of 40 days. Here, CDK8 is linked to neoplasm.