Plus-end directed microtubule-dependent motor protein which displays processive activity (PubMed:27117409, PubMed:10225949). Is involved in regulation of microtubule dynamics, synapse function and neuronal morphology, including dendritic tree branching and spine formation (PubMed:27117409). Plays a role in lerning and memory (PubMed:27117409). Involved in delivery of gamma-aminobutyric acid (GABA(A)) receptor to cell surface (PubMed:25172774). Interacts with TRIM3; the interaction positively affects motility of KIF21B (PubMed:24086586). Interacts with GABARAP and GABA(A) receptor subunits: GABRG2, GABRA1 and GABRA2 (By similarity). May interact with GABA(A) receptor subunits: GABRB2 and GABRB3 (By similarity). Expressed in brain (at protein level) (PubMed:27117409, PubMed:25172774). Expressed in spleen and at lower levels in testes (PubMed:10225949). Mutant mice exhibit behavioral changes involving learning and memory deficits. Belongs to the TRAFAC class myosin-kinesin ATPase superfamily. Kinesin family.