Inclusion Criteria:

          -  Participants must have a histologically confirmed HER2 positive breast cancer (by
             ImmunoHistoChemistry (IHC) 3+ or fluorescence in situ hybridization (FISH) ratio ≥
             2.0). Phase 1: unresectable locally advanced or metastatic breast cancer. Phase 2:
             clinical stage 1-3 early stage breast cancer with primary tumor is at least 1cm
             measured by clinical exam or by radiologic breast imaging tests.

          -  Prior Therapy - Phase 1: Must be candidates to receive paclitaxel chemotherapy in
             combination with trastuzumab and pertuzumab. Phase 2: No prior chemotherapy,
             radiation, or definitive therapeutic surgery (e.g., mastectomy, lumpectomy or axillary
             dissection) for this malignancy. Patients who have had a prior sentinel lymph node
             biopsy for this malignancy are eligible. Patients who received equal to or less than 1
             cycle of therapy (up to 4 weeks) will be allowed to enroll in this trial.

          -  Patients who received tamoxifen or another selective estrogen receptor modulator
             (SERM) for the prevention or treatment of breast cancer or for other indications
             (e.g., osteoporosis, prior ductal carcinoma in situ (DCIS)), or who receive aromatase
             inhibitors for prevention or treatment of breast cancer, are eligible. Patients who
             are hormone-receptor positive and who have received other hormonal agents for the
             treatment of breast cancer (e.g., Fulvestrant®) are also eligible. Tamoxifen therapy
             or other hormonal agents should be discontinued at least 1 week before the patient is
             started on study therapy.

          -  Age ≥ 18 years.

          -  Eastern Cooperative Oncology Group (ECOG) performance status 0 or 1.

          -  Must have normal organ and marrow function within 2 weeks of registration (except
             where specified otherwise).

          -  Women of childbearing potential must agree to use adequate contraception (hormonal or
             barrier method of birth control) prior to study entry and for the duration of study
             participation.

          -  Ability to understand and willingness to sign a written informed consent document.

        Exclusion Criteria:

          -  Receiving any other investigational agents during protocol therapy, or up to 14 days
             or 5 half-lives (whichever is longer) prior to beginning protocol therapy. There
             should be a least a 1-week interval between last dose of endocrine therapy and
             protocol therapy.

          -  Have had chemotherapy or radiation therapy within 2 weeks prior to beginning protocol
             therapy.

          -  Uncontrolled intercurrent illness including, but not limited to, ongoing or active
             infection, congenital prolonged QT syndrome, symptomatic congestive heart failure,
             unstable angina pectoris, cardiac arrhythmia, or psychiatric illness/social situations
             that would limit compliance with study requirements.

          -  Current use of corticosteroid therapy > 5 mg/day of prednisone or equivalent doses of
             other corticosteroids (topical, intranasal, and inhaled corticosteroids in standard
             doses and physiologic replacement for participants with adrenal insufficiency are
             allowed).

          -  Known active or symptomatic central nervous system (CNS) metastases and/or
             carcinomatous meningitis. Asymptomatic, treated, and/or stable brain metastases, as
             measured by subsequent radiologic evaluations at least two months apart, are
             permitted.

          -  Pregnant or breast feeding.

          -  Known HIV-positive.

          -  Known current or a history of hepatitis B or C virus, including chronic and dormant
             states, unless disease has been treated and confirmed cleared.

          -  Major surgery within 4 weeks of initiation of study drug.

          -  Second invasive malignancy requiring active treatment.