IMPORTANT study is a multicenter, open-label, prospective, randomized-controlled, non-inferiority trial with a pragmatic approach involving older patients (≥ 70 years old) with advanced hormone receptor (HR)-positive/human epidermal growth factor receptor 2 (HER2)-negative breast cancer, not amenable for curative treatment and without prior therapy for advanced disease, who are suitable to receive CDK 4/6-inhibitors plus endocrine therapy as first line therapy. The study implements two approaches with high level of evidence, namely the use of comprehensive geriatric assessment (CGA) approach in treatment decision making and the use of CDK 4/6-inhibitors as the initial treatment of choice, to investigate whether a common clinical practice (starting dose reduction of CDK 4/6-inhibitors in older patients) with evidence of low certainty can be standardized using a more individualized-based approach. On the basis of baseline CGA assessment, patients will either receive full dose of CDK 4/6-inhibitors plus endocrine therapy (if patients are fit according to CGA) or be randomized to full dose vs. reduced initial dose of CDK 4/6-inhibitors (if vulnerable or frail according to CGA). The study hypothesis is that adjusting the dose according to vulnerability will allow patients to tolerate treatment better without jeopardizing the treatment efficacy. This project has received funding from the European Union's HORIZON 2022 research and innovation actions supporting the implementation of the Mission on Cancer under grant agreement No 101104589.