HER2 is overexpressed in certain types of cancer, and the availability of both HER2‐high and HER2‐low cell lines, together with the clinical success of high drug‐to‐antibody ratio (DAR) trastuzumab conjugates, such as Enhertu (DAR 7.7), which also utilizes CatB–mediated drug release, makes HER2 an ideal model system for evaluating targeted CO delivery.[35, 61] Achieving detectable intracellular CO concentrations requires both high DAR loading and high HER2 receptor expression. The gene discussed is ERBB2; the disease is cancer.