The high incidence of ErbB2 positive non-invasive DCIS together with multiple studies showing the incapability of ErbB2 to induce invasion of non-cancerous MCF10A mammary epithelial cells have led into a “two-hit” theory of invasion, where overexpression of the ErbB2 oncogene is the first hit, which will only promote invasive cancer after receiving the second, additional hit. The gene discussed is ERBB2; the disease is ductal breast carcinoma in situ.