ERBB2 and breast cancer: HER2 is overexpressed in 20% of breast cancers (but overall is expressed in 60% of breast cancers above the healthy breast tissue level [42], thus not restricted only to HER2-positive tumours) and together with MGB1 show in a numbers of studies clinical validity in detection of breast cancer CTCs [31,43,44,45,46,47,48,49,50,51].