Overall, our genetically matched mice have revealed that 4R NM hTau over expression is pathogenic in a manner distinct from classical aging-related tauopathy, underlining the importance of assaying the effects of transgenic disease-related proteins at appropriate stages in life.<b>SIGNIFICANCE STATEMENT</b>Due to differences in creation of transgenic lines, the pathological properties the P301L mutation confers to the tau protein <i>in vivo</i> have remained elusive, perhaps contributing to the lack of disease-modifying therapies for tauopathies.