Aberrant ErbB2 activation is found in about 25%–30% of invasive ductal breast carcinomas, however, up to 85% of mammary ductal carcinomas in situ (DCIS), which are early non-invasive mammary carcinomas, also overexpress ErbB2 [87,88]. Here, ERBB2 is linked to ductal breast carcinoma in situ.