0_9217 Inheritance of genetic material relies on the interaction between kinetochores built on centromeric DNA, and microtubules of the mitotic spindle. Properly formed connections are stabilized, and microtubules are prevented from shortening; incorrect attachments are destabilized and broken. The mechanisms ensuring stabilization of a subset of kinetochore-attached microtubules are poorly understood at the molecular level. In humans, each kinetochore recruits hundreds of copies of the Ndc80 complex that crosslinks kinetochores and microtubule ends. Multivalency of the Ndc80 is necessary for its function: monomeric Ndc80 does not bind to microtubule ends. Here we demonstrate that in order to stabilize microtubule ends against mechanical force, Ndc80 complexes within a microtubule-bound oligomer need to interact with each other in a cooperative manner. We describe the internal loop region within the Ndc80 coiled-coil as the site of such cooperativity, and dissect the effect of the loop-mediated homotypic interactions between neighboring Ndc80s on single-molecule properties of the Ndc80 complex, on its ability to stabilize microtubule ends under force, and on cell cycle progression in vivo. These results allow us to re-evaluate the mechanisms of the force-sensing within the kinetochore.