ESR1 and neoplasm: As overall immunohistochemical ErbB4 expression was associated with tumor ER-positivity (P = 0.001) and a high histological grade of differentiation (P = 0.004) in the same series, consistent with several previous reports [34]–[37], these findings imply that factors such as molecules promoting ErbB4 shedding may specifically interfere with the prognostic significance of serum ErbB4.