CDK4 and cancer: Materials and Methods: This prospective single-center study utilized four internationally validated questionnaires: the EORTC QLQ-C30 (European Organization for Research and Treatment of Cancer Core Cancer Quality of Life Questionnaire), the Depression, Anxiety, and Stress Scale-21 (DASS-21), the Multidimensional Fatigue Inventory (MFI), and the Pittsburgh Sleep Quality Index (PSQI), administered to a cohort of 76 patients undergoing treatment for metastatic breast cancer with CDK4/6 inhibitors.