Tau is a protein that is found mostly in the neurons of the central nervous system. They help hold together the "microtubules" within the neurons. and when changes happen in the way the tau proteins are supposed to work the microtubules break apart. The tau proteins which are no longer holding the microtubules together form strands called "fibrils", which then clump together inside the neuron to make what are called neurofibrillary tangles . These clumps, also known as 'tau tangles', are all that remain after a neuron has died. 