Microtubule Associated Proteins, or MAPS, bind to the tubulin subunits of microtubule structures and regulate their functional stability. In the cell MAPs bind to monomer and multimerized tubulin. MAP binding to multimerized tubulin further stabilizes the formation of higher order microtubulin structures. MAP binding to microtubule structures is mediated through phosphorylation through Microtubule Affinity Regulated Kinase (MARK). Phosphorylation releases MAPs bound to microtubules, destabilizing the structure, driving it toward disassembly. There are predominately two MAP types, I, II. Type II MAP includes MAP2, MAP4, and tau and are found in nervous tissue. Six tau isoforms exist in brain tissue, and they are distinguished by their number of binding domains. Three isoforms have three binding domains and the other three have four binding domains. The binding domains are located in the carboxy-terminus of the protein and are positively-charged (allowing it to bind to the negatively-charged microtubule). The isoforms with four binding domains are better at stabilizing microtubules than those with three binding domains.