Immunohistochemical staining of ALDH1 was applied to 21 cases with benign breast diseases, 47 ductal carcinoma in situ (DCIS) cases, 62 cases diagnosed with invasive cancer with extensive intraductal component (EIC), and 58 cases diagnosed with invasive cancer without EIC. The gene discussed is ALDH1A1; the disease is ductal breast carcinoma in situ.