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KIF1A/unc104 is the founding member of kinesin-3 family and transports synaptic vesicle precursors in neuronal axons.Previous studies have demonstrated that a single monomer of KIF1A/unc104 can move along a microtubule without full detachment by using the diffusive anchoring via the loop 12 (K-loop) of kinesin motor domain.Clustering of KIF1A/unc104 on the lipid membrane facilitates the oligomerization, which has been shown to convert KIF1A/unc104 to a processive motor that can resist against external loads up to about 6 pN.However, it has been unclear whether a single forced dimer of KIF1A/unc104 is processive under load, in light of the recent analyses of the mixed motor gliding assays (Arpag et al., Biophys J 2014).We have, therefore, reexamined the motility of unc104 forced dimer under the same conditions reported previously.Although a single forced dimer of unc104 showed robust unidirectional plusend directed runs as reported, the beads coated with low surface density of unc104 dimer generated force only about 1-2 pN.The previously reported stall force of 6 pN was achieved only when the beads were coated with much higher density of unc104 dimer.These results suggest that cooperation among multiple unc104 proteins would be required for the processive run against external load, and that cluster formation rather than the monomer-dimer conversion would be essential for the physiological regulation of the transport properties of KIF1A/unc104.