*1L CDK4/6i treatment duration was defined as the time from the start of the specific CDK4/6i to discontinuation of treatment or death during the 1L. Patient data were derived from the Flatiron Health US real-world oncology database of de-identified, longitudinal electronic health record data from >800 sites of care, including >721,000 patients with breast cancer.^1-3
†Subsequent treatment was defined as a change in systemic therapy triggering a line advancement, with the exception of changes in AI (i.e., a change from one AI to another did not trigger a line advancement). Subsequent treatments for mBC were examined in patients who started 1L treatment in 2017 onward, when the three CDK4/6is were all approved in the US.
‡CDK4/6i switching was defined as changing CDK4/6i with or without changing the AI partner. 

*The balance in these baseline characteristics was assessed using a standardised mean differences approach, with values ≥0.1 indicating a non-negligible imbalance.¹ Standard difference data not shown. More patients were eligible for the palbociclib cohort, as palbociclib was the first CDK4/6i approved in the US (in 2015), followed by ribociclib (in 2017) and abemaciclib (in 2018).^{1, 4-6} 
†Visceral disease is defined as metastatic disease in the lung and/or liver. Patients could have had other sites of metastases.⁷
‡Bone-only disease is defined as metastatic disease in the bone only.⁷