Model for Num1-mediated activation of dynein-mediated spindle positioning. Our data suggest that at the moment of offloading (step 1), contact between dynein–dynactin and cortical Num1 triggers a cascade of events that ultimately leads to Pac1 dissociation (step 3); however, the MTBD (deletion of which interrupts this process) is required to make contact with the microtubule to initiate Pac1 dissociation (step 2).