AimsA recent study examining the specificity of human epidermal growth factor receptor (HER) 2 pharmacodiagnostic antibodies demonstrated that CB11 and 4B5 stain both HER2‐transfected and HER4‐transfected cell lines. However, there has been no evidence showing that 4B5 has affinity for HER4 in clinically obtained tissues, and, if so, whether this has any impact on the assessment of HER2. We therefore sought to determine the expression of membrane‐bound HER4 in clinical breast carcinomas, and evaluate its impact on the clinical utility of 4B5 in determining HER2 status.Methods and resultsBreast carcinomas were assessed by immunohistochemistry (IHC) for membrane‐bound HER4 using anti‐HER4 clone E200. HER2 expression in these cases was then assessed using anti‐HER2 clone 4B5, and a reference clone, SP3. In all 117 membrane HER4‐positive cases (out of 241), 4B5 scored equal to or less than the reference anti‐HER2 clone SP3. Eighteen cases were positive for membrane‐bound HER4 by E200 and negative by 4B5, including a membrane HER4 level 3+ case.ConclusionsNo cross‐reactivity of 4B5 with membrane‐bound HER4 was identified in the clinical IHC analysis of formalin‐fixed paraffin‐embedded breast carcinoma cases as evidenced by the HER4 antibody clone E200.