Required for determination of left/right asymmetry in nervous system. Acts together with unc-40 to control an initial left-right asymmetric polarization of the Q neuroblasts. Mig-21 and unc-40 may control the asymmetry in Wnt signaling response by restricting posterior polarization to one of the 2 Q neuroblasts. Involved in left-side QL posterior migration. In right-side QR, unc-40 and mig-21 pathways mutually inhibit each other in posterior migration, allowing anterior QR migration. Glycosylated via C-mannosylation by dpy-19 at Trp-58 and Trp-61.