In addition, data from the phase 3 MONARCH 2 clinical trial showed trends in CFS indicating abemaciclib/fulvestrant delayed the need for subsequent chemotherapy compared to placebo/fulvestrant (median CFS, intent-to-treat population: 25.5 months [16]), on par with the median CFS following CDK4 and 6 inhibitor therapy observed in the current study (26.1 months). Here, CDK4 is linked to myalgic encephalomeyelitis/chronic fatigue syndrome.