Modulation of CCNE1 expression in HER2-positive cancer cell lines regulated sensitivity to the HER2-targeting antibody–drug conjugate trastuzumab deruxtecan (T-DXd) in vitro, and adavosertib acted synergistically with T-DXd in HER2-expressing patient-derived xenografts in vivo (41). The gene discussed is CCNE1; the disease is cancer.