We show that CCND1 amplification or overexpression is associated with chemotherapy resistance in both ER+ and TN breast tumors: we observed a significant enrichment of CCND1-amplified tumors in our cohort of chemotherapy resistant samples, CCND1 amplification was associated with higher doxorubicin IC50s in breast cancer cell lines, and expression of CCND1 was significantly higher in tumors that did not achieve a pCR in our validation cohort. Here, CCND1 is linked to breast cancer.