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Current evidence from both randomized trials and real-world studies suggests that older patients with advanced hormone receptor-positive/HER2-negative (HR+/HER2) breast cancer derive clinical benefit from the addition of CDK4/6 inhibitors to endocrine therapy. However, a higher risk for adverse events due to CDK4/6 inhibitors among older patients is evident, leading to a trend of initiating CDK4/6 inhibitors at lower dose in clinical practice, though without evidence. The aim of the IMPORTANT-trial, a pragmatic, multinational, open-label, partly decentralized randomized trial is to investigate whether lower starting dose of CDK4/6 inhibitors combined with endocrine therapy is comparable to full dose in older (>= 70 years old) patients with advanced HR+/HER2- breast cancer who are assessed as vulnerable or frail based on comprehensive geriatric assessment.