DECRESCENDO is a multicentre, open-label, dual-phase single-arm phase II de-escalation study evaluating neoadjuvant treatment with 12 administrations of weekly IV paclitaxel 80 mg/m2 (or IV docetaxel 75 mg/m2 every 3 weeks for 4 cycles) combined with subcutaneous (SC) fixed dose combination (FDC) of pertuzumab and trastuzumab (loading dose of 1200 mg pertuzumab and 600 mg trastuzumab, followed by 600 mg pertuzumab and 600 mg trastuzumab) every 3 weeks for 4 cycles. Surgery will be performed according to local guidelines in all subjects after neoadjuvant treatment. After surgery, subjects who achieve a pCR (defined as pT0/Tis pN0) will receive adjuvant pertuzumab and trastuzumab FDC SC for additional 14 cycles. Subjects with residual invasive disease will receive salvage adjuvant trastuzumab emtansine (T-DM1, 3.6 mg/kg, IV every 3 weeks) for 14 cycles. In subjects whose residual invasive disease is classified per RCB score as ≥2, 3 to 4 cycles of anthracycline-based chemotherapy may be administered, at the investigator's discretion, before the 14 cycles of T-DM1. If histopathological analysis finds that the surgical specimen from a subject with residual disease is ER-positive and/or PR-positive, adjuvant endocrine therapy may be administered concomitantly with study treatment, at the investigator's discretion and according to local guidelines. Adjuvant radiotherapy will be mandatory after breast-conserving surgery, whereas it will be performed according to local guidelines after mastectomy, and it will be administered concomitantly with pertuzumab and trastuzumab FDC SC in subjects who achieve a pCR, and concomitantly with T-DM1 in subjects with residual invasive disease (after anthracycline-based chemotherapy in subjects assigned to receive this treatment).