AbstractTuberous sclerosis complex (TSC) is a neurodevelopmental disorder caused by mutations in the TSC1 and TSC2 genes and autosomal dominantly inherited. These mutations cause hyperactivation of the mammalian Target of Rapamycin (mTOR) pathway, leading to the development of nonmalignant masses involving various organ systems. Patients with TSC also experience neuropsychiatric symptoms collectively termed Tuberous Sclerosis Complex Associated Neuropsychiatric Disorder (TAND). Due to research advancements in TSC, patients now live well beyond the age of 50. Many experience objective impairment of memory and executive function, supported by formal neuropsychological testing, beginning in their late 40s. Biomarker analysis has described elevated levels of phosphorylated tau-181 in the cerebrospinal fluid of patients with TAND. Tau-PET imaging has also shown focal accumulation of the radiotracer flortaucipir (AV1451), suggesting that TSC may be a neurodegenerative disorder arising from accumulation of phosphorylated tau. However, the flortaucipir tracer has been reported to have significant off-target binding, preventing definitive conclusions from being drawn about the molecular etiology of neurodegeneration in TSC. Therefore, we initiated the Colocalization of AV1451 and Phosphorylated Tau in Adult brain tissue (CAPA) study. This study aimed to determine if flortaucipir is bound to phosphorylated tau in brains of patients with TSC and further sought to determine the specific tau isoform seen in TSC. Our results show that flortaucipir labels the 3R/4R isoform of phosphorylated tau, commonly seen in Alzheimer’s disease. However, amyloid staining was negative in brains of adult patients with TSC. Therefore, we conclude that TAND symptoms are due to the accumulation of the phosphorylated tau isoform seen in Alzheimer’s disease. This study suggests that hyperactivation of the mammalian Target of Rapamycin pathway may play a role in the amyloid-independent development of 3R/4R tau aggregation. Our findings could lead to a new era of anti-tau therapies used to treat both disorders.