PDGFR (PDGFRa or PDGFRb)‐specific deletion in CD34+ cells in vivo revealed that CD34+ cells give rise to Periostin+ myofibroblasts through PDGFRb and not PDGFRa, whereas both receptors are required for CD34+ cell proliferation in AAA. The gene discussed is PDGFRA; the disease is triple-A syndrome.