On a PET scan, amyloid shows up as a wash of colour across the cortex, brighter where the sticky protein has gathered into plaques. For decades the reading of those scans has rested on a tidy assumption: carry the APOE ε4 gene variant, the strongest common genetic risk factor for late-onset Alzheimer’s, and your brain is far more likely to light up. More amyloid, more risk New! Sign up for our email newsletter on Substack.