ERBB2 and breast cancer: HER2 overexpression is found in approximately 20% of breast cancers [31-32] and is associated with breast-to-brain metastasis in nearly half of patients with this tumor subtype [5, 33]. Discordance in HER2 status, in which the primary tumor is negative for HER2 while the brain metastasis is HER2-positive, has been found in up to 24% of cases, and this is associated with decreased survival [34-35].