We found that non‐BM CD34+ cell‐eliminated mice exhibited a higher incidence of Ang II‐induced AAA, more pronounced aortic dilatation, thinner adventitial layer, and fewer tdT+/Periostin+ myofibroblasts in the aneurysmal aorta than BMT‐CTRL mice (Figure S13, Supporting Information). The gene discussed is POSTN; the disease is triple-A syndrome.