Neurofibrillary tangles (NFTs) are aggregates of hyperphosphorylated tau protein that are most commonly known as a primary biomarker of Alzheimer's disease. Their presence is also found in numerous other diseases known as tauopathies. Little is known about their exact relationship to the different pathologies. Formation Neurofibrillary tangles are formed by hyperphosphorylation of a microtubule-associated protein known as tau, causing it to aggregate, or group, in an insoluble form. (These aggregations of hyperphosphorylated tau protein are also referred to as PHF, or "paired helical filaments"). The precise mechanism of tangle formation is not completely understood, and it is still controversial whether tangles are a primary causative factor in disease or play a more peripheral role. Cytoskeletal changes Three different maturation states of NFT have been defined using anti-tau and anti-ubiquitin immunostaining. At stage 0 there are morphologically normal pyramidal cells showing diffuse or fine granular cytoplasmic staining with anti-tau. In other words, cells are healthy with minimal tau presence; at stage 1 some delicate elongate inclusions are stained by tau antibodies (these are early tangles); stage 2 is represented by the classic NFT demonstration with anti-tau staining; stage 3 is exemplified by ghost tangles (tangles outside of cells where the host neuron has died), which are characterized by a reduced anti-tau but marked anti-ubiquitin immunostaining. Causes Mutated tau The traditional understanding is that tau binds to microtubules and assists with their self-assembly, formation and stabilization. However, when tau is hyperphosphorylated, it is unable to bind and the microtubules become unstable and begin disintegrating. The unbound tau clumps together in formations called neurofibrillary tangles. More explicitly, intracellular lesions known as pretangles develop when tau is phosphorylated excessively and on improper amino acid residues. These lesions, over time, develop into filamentous interneuronal neurofibrillary tangles (NFTs) which interfere with numerous intracellular functions. Seeking a reliable animal model for tau-related pathologies, researchers expressed the human mutant P301L tau gene in adult mice. This experiment resulted in the formation of neurofibrillary tangles and pretangle formations. The human mutant P301 tau gene is associated with frontotemporal dementia with parkinsonism, another tauopathy associated with NFTs. It was found that the degree of tau pathology was dependent on time and the level of gene expression. Groups receiving a combination of a promoter and enhancer in the vector saw increased tau expression, as early as 3 weeks after vector injection, which was measured using a Western blot. These groups also