MAPT and Alzheimer disease: The interaction of pathogenic proteins also plays an important role here, as they may lead to the development of a specific disease (for instance, interaction of Aβ and tau results in AD) or to the propagation of concomitant pathologies that aggravate the progression of the main disease (TDP-43 or α-Syn pathology in AD or Aβ pathology in PD), or to the appearance of another distinct disease in addition to the main one (AD at the background of T2DM and vice versa).