This study is exploring a new treatment combination for advanced breast cancer that is **ER-positive** (Estrogen Receptor-positive) and **HER2-negative** (Human Epidermal growth factor Receptor 2-negative). It combines two medications: camizestrant and ribociclib. **Advanced breast cancer** means the cancer has spread beyond the breast. The study is **Phase IIIb**, which means researchers are checking how well the treatment works and its safety in a large number of people. **Key Points:** - The study includes about 150 patients who haven't had other treatments for their advanced cancer. - Participants will take daily pills of both drugs until they stop the study treatment. - Eligibility includes being 18 or older, having a specific type of breast cancer, and agreeing to use birth control if needed. The study helps determine if this drug combination can effectively treat this type of breast cancer. If you're interested, consider discussing it with your doctor to see if it's a good fit for you. You must meet certain health criteria and agree to follow study guidelines.