CDK4 and breast cancer: In ER-positive early BC patients with detectable ctDNA after surgery via the highly sensitive Signatera mutation assay, the investigator-initiated, multi-center, Phase II randomized LEADER trial (NCT03285412) evaluates the use of endocrine therapy with and without Ribociclib, a CDK4/6 inhibitor, in the adjuvant setting regarding the ctDNA clearance after one year and as secondary endpoint also regarding DFS.