Given that disrupted dimerization significantly enhanced HACE1 activity, we surmised that misregulation of HACE1 activity by cancer‐associated mutations may lead to abnormal substrate binding and ubiquitination activity.[8, 15, 32] To this end, we first examined the binding of the target substrate RAC1 to HACE1 and used amide HDX–MS to map the RAC1 interaction sites in the HACE1–RAC1 complex. This evidence concerns the gene HACE1 and cancer.