MAPT and Alzheimer disease: Previous studies have demonstrated associations between individual CSF biomarkers (for the AD biomarkers Aβ, T‐tau, and P‐tau) and ventricle volume (likely dilution effects; Edsbagge et al, 2017; van Waalwijk van Doorn et al, 2017), but to our knowledge, this is the first large‐scale study of such associations, and how they affect associations between CSF biomarker and genetic variants.