APOE and Alzheimer disease: For example, the lack of blood stream in the current organoid system prevents us from modeling the effects resulted from serum exposure resulted from the blood–brain barrier (BBB) leakage, a well‐known risk factor for AD, which can be caused by aging,[12] the greatest risk factor for AD, and apolipoprotein E4 (ApoE4),[13] a well identified AD risk gene.[2, 14]