The FDA approved camizestrant (Etcamah) for ESR1-mutated HR+/HER2– advanced breast cancer, enabling ctDNA-guided treatment switching. The approval is linked to data discussed in connection with the SERENA-6 study and positions clinicians to monitor for ESR1 resistance mutations in circulating tumor DNA before clinical progression. In an interview, Dana-Farber’s Erica L. Mayer described how an early switch to camizestrant plus a CDK4/6 inhibitor once an ESR1 mutation emerges in ctDNA improved progression-free survival and delayed deterioration in quality of life versus continuing aromatase inhibitor-based therapy. The decision also reinforces a broader clinical-trial and practice framework built around molecular progression triggers rather than waiting for symptoms or radiographic decline.