Besides the ACE2-Ang-(1-7)-MasR/AT2R axis, we further explored the role of Ang IV in Ang II-induced abdominal aortic aneurysm (AAA) in apolipoprotein E-knockout (ApoE-/-) mice and found dose-dependent bidirectional effects of Ang IV: a medium-dose Ang IV (1.44 mg/kg daily) significantly reduced the incidence and severity of AAA, whereas a high-dose Ang IV (2.88 mg/kg daily) had no protective effect. The gene discussed is APOE; the disease is triple-A syndrome.