Inclusion Criteria:

          -  HER2-positive Stage I histologically confirmed invasive carcinoma of the breast

          -  ER/PR determination is required

          -  HER2 positive, confirmed by central testing: IHC 3+, FISH HER2/CEP17 <2.0 with an
             average HER2 copy number >/=6.0, or FISH HER2/CEP17 >/= 2.0

          -  Bilateral breast cancers that individually meet eligibility criteria are allowed

          -  Subjects with multifocal or multicentric disease are eligible as long as each tumor
             individually meets eligibility criteria

          -  Subjects with a history of ipsilateral DCIS are eligible if they were treated with
             wide-excision alone, without radiation therapy; Patients with a history of
             contralateral DCIS are not eligible.

          -  Should have tumor tissue available and a tissue block of sufficient size to make 15
             slides, which must be sent to a DFCI site for testing

          -  Less than or equal to 90 days since most recent breast surgery for this breast cancer

          -  All tumor should be removed by either a modified radical mastectomy or a segmental
             mastectomy (lumpectomy) with either a sentinel node biopsy or axillary dissection

          -  All margins should be clear of invasive cancer or DCIS

          -  May have received up to 4 weeks of tamoxifen therapy or other hormonal therapy, for
             adjuvant therapy for this cancer

          -  Prior oophorectomy for cancer prevention is allowed

          -  Subjects who have undergone partial breast radiation (duration </= 7 days) prior to
             registration are eligible. Partial breast radiation must be completed prior to 2 weeks
             before starting protocol therapy.

          -  Must have discontinued any investigational drug at least 2 weeks prior to
             participation

          -  Willing to use one highly effective from of nonhormonal contraception or two effective
             forms of nonhormonal contraception while on study and for 7 months after end of study
             treatment

          -  Subjects undergoing lumpectomy must have no contraindications to radiation therapy

        Exclusion Criteria:

          -  Pregnant or breastfeeding

          -  Use of potent CYP3A4 inhibitors during the study treatment period

          -  Excessive alcohol intake (more than 3 alcoholic beverages per day)

          -  Locally advanced tumors at diagnosis

          -  History of previous invasive breast cancer

          -  History of prior chemotherapy in the past 5 years

          -  History of prior trastuzumab or prior paclitaxel therapy

          -  Active, unresolved infection

          -  Active liver disease

          -  History of a different malignancy except for the following: disease free for at least
             5 years and at low risk for recurrence; cervical cancer in situ, basal or squamous
             cell carcinoma of the skin

          -  Active cardiac disease