ESR1 and neoplasm: Pre-incubation with 1 μmol/l seribantumab, on the other hand, resulted in complete inhibition of ErbB3 phosphorylation, partial inhibition of Akt phosphorylation, and reduction or complete inhibition of ER phosphorylation on Ser305 and Ser167, respectively.33 On the basis of these data, we sought to determine if ErbB3 signaling is active in MCF-7Ca-based xenograft models, and if blocking this pathway affects tumor growth.