This research study is for people with a type of breast cancer called hormone receptor-positive, HER2-positive. This means the cancer grows from hormones and a protein called HER2. After surgery, the study uses two medicines, trastuzumab and pertuzumab, which are given together as one injection called PHESGO. These medicines are special proteins that help the body fight cancer. They are already approved by the U.S. Food and Drug Administration (FDA) for treating this type of cancer.

The study also uses hormonal therapy to lower the hormones that make cancer grow. Participants will receive PHESGO treatment for 1 year and hormonal therapy for about 5 years. The study checks if this combination works well and is safe.

• The study lasts about 5 years, with PHESGO treatment for 1 year.
• 375 people are expected to join this study.
• Participants must have had surgery to remove the cancer.

If you join, you will have medical tests and follow-up visits. It's important to be willing to stop using hormonal birth control and use other forms of contraception during the study. If you meet the eligibility criteria and are interested, you can talk to your doctor about participating.