According to the recommendations of the International Working Group, an amyloid-pathology-positive and tau-pathology-negative biomarker profile in a patient with an atypical AD phenotype may account for a possible—as opposed to a (highly) probable—AD diagnosis as the primary diagnosis: “The clinician could deem that Alzheimer’s disease is not the dominant pathology driving the clinical phenotype but only a co-pathology” [63]. The gene discussed is MAPT; the disease is early-onset autosomal dominant Alzheimer disease.