In Reply In our recently published study,1 we have shown an association of detection of circulating tumor DNA (ctDNA) in follow-up after treatment of breast cancer with a high risk of relapse. The results showed a substantial lead time over clinical relapse of 10.7 months overall and extending to 13.3 months in estrogen receptor–positive, ERBB2-negative breast cancer. This provides evidence for clinical validity of ctDNA molecular relapse assessment. However, our study does not provide evidence of clinical utility, which could only be generated from randomized clinical trials that assess whether treating patients on the basis of ctDNA assessment can improve outcome.