MAPT and Alzheimer disease: Additionally, biosensor cells expressing 3R, 4R or both 3R and 4R tau constructs have also indicated tau-based disease selectivity of cell infection with 4R tauopathies infecting only those cells expressing the 4R construct, 3R tauopathies infecting only 3R-expressing cells, and AD and CTE cases only initiating aggregation with the expression of both 3R and 4R tau or alternatively, overexpression of the 4R construct [63].