MAPT and early-onset autosomal dominant Alzheimer disease: Interestingly, although full-length tau is found in CSF from healthy humans, a heterogeneous population of fragments—including the NH2-terminal and/or prolin-rich domain—is mainly discernible in Alzheimer’s disease patients (Johnson et al., 1997; Portelius et al., 2008; Meredith et al., 2013; Amadoro et al., 2014; Chen et al., 2019; Cicognola et al., 2019).