Glycogen synthase kinase 3 (GSK-3) is a ubiquitously expressed enzyme that is constitutively active in resting cells and phosphorylates a wide variety of proteins — from glycogen synthase to microtubule-associated proteins. During insulin signalling, the kinase PDK1 phosphorylates and activates protein kinase B (PKB), which in turn phosphorylates and inactivates GSK-3. James Wakefield and co-workers now show that a similar pathway controls spindle dynamics at mitosis (see p. 637). They find that GSK-3 is present along spindle microtubules. In addition, they demonstrate that the phosphorylated form is concentrated at spindle poles and centrosomes. where phosphorylated, active PKB is also present. Using two distinct GSK-3 inhibitors, the authors show that inhibition of GSK-3 leads to an increase in the length of spindle microtubules and defective chromosome alignment at prometaphase. Furthermore, live microscopy reveals that this reduces chromosome movement and produces a prometaphase arrest. Wakefield and co-workers conclude that spatiotemporal control of GSK-3 activity is important for control of microtubule dynamics at mitosis. It could ensure stabilization of microtubules at centrosomes but allow them to remain dynamic elsewhere and thus scour the cytoplasm for chromosomes.