An inverse relationship between endocrine therapy responsiveness and ErbB2 expression has not been observed in all studies. This may be due to discordant ER status between the primary tumor and metastatic sites. ER status can be discordant in approximately 20% of cases, with a tendency for metastatic disease to become ER negative with time. Data concerning ErbB2 is more limited, but there may be similar lack of concordance between primary tumor and metastases.