To determine whether cholesterol increases in AD cells occurred via upregulation of the de novo cholesterol synthesis pathway, we quantified the activity of the 3‐hydroxy‐3‐methylglutaryl‐CoA reductase (HMGCR), the rate‐limiting enzyme in the synthesis of cholesterol, in γ‐secretase‐deficient cells and controls. The gene discussed is HMGCR; the disease is Alzheimer disease.