Chromosomes are segregated to daughter cells by a microtubule-based molecular machine, the mitotic spindle. The spindle has two poles, each one carrying an exact complement of chromosomes to each daughter cell. Spindle morphogenesis requires spatially controlled microtubule nucleation. The proteins required for nucleation are known, but they are surprisingly inactive in isolated form. We are studying how these proteins are activated to nucleate microtubules during spindle formation and how their activation is regulated.