APP and Alzheimer disease: To test this, we measured the concentration of cholesterol in homogenates and subcellular fractions from mouse embryonic fibroblasts (MEFs) null for both PSEN1 and PSEN2 (PS‐DKO) (Herreman et al, 2000) [these cell display high levels of C99 in MAM (Pera et al, 2017)] (Fig 1A) and from homogenates of AD fibroblasts (Fig EV1A) by liquid chromatography–mass spectrometry (LC‐MS) (Chan et al, 2012).