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A needle biopsy of a mass in the right breast of a 36-year-old woman revealed invasive ductal carcinoma (IDC), and approximately 20% of cancer cells showed unequivocal membranous staining with the HercepTest. After systemic therapy with trastuzumab and paclitaxel followed by FEC (fluorouracil + epirubicin + cyclophosphamide), a right mastectomy was performed. By histological and immunohistochemical examinations, the resected tumor consisted mainly of E-cadherin-negative invasive lobular carcinoma (ILC), and the rest was ERBB2-positive IDC; thus, the diagnosis was mixed ductal and lobular carcinoma. Multiplex ligation-dependent probe amplification and fluorescencein situhybridization (FISH) analyses revealed that ILC and IDC shared high-level amplification ofCCND1in homogeneously staining regions (HSR) and that IDC had an additional HSR-type amplicon ofERBB2. These findings strongly indicate that IDC and ILC had a common precursor cell withCCND1amplification. Review of the biopsy specimen with FISH showed IDC with gene amplifications ofCCND1andERBB2as a minor component, IDC without amplification ofCCND1orERBB2as a major component, and a minute portion of ILC withCCND1amplification. We speculate that chemotherapy and trastuzumab caused a marked reduction in IDC; however, ILC withCCND1amplification was resistant to chemotherapy and grew.