- Participants will enroll in this study at the time they are starting their adjuvant therapy for breast cancer. Participants will receive chemotherapy with paclitaxel every week for 12 weeks. They will begin to receive trastuzumab at the same time they begin paclitaxel. Once they have completed the 12 weeks of paclitaxel and trastuzumab, they will receive trastuzumab every 3 weeks or weekly for 40 weeks.