C-mannosyltransferase that mediates C-mannosylation of tryptophan residues on target proteins such as unc-5 and mig-21. Mediates the attachment of alpha-mannose in C-C linkage to the C2 of the indole ring of tryptophan. C-mannosylation takes place in the endoplasmic reticulum and frequently found in thrombospondin (TSP) type-1 repeats and in the WSXWS motif of type I cytokine receptors. Required to orient neuroblasts QL and QR correctly on the anterior/posterior (A/P) axis: QL and QR are born in the same A/P position, but polarize and migrate left/right asymmetrically, QL migrates toward the posterior and QR migrates toward the anterior. Required with unc-40 to express mab-5 correctly in the Q cell descendants (By similarity). Belongs to the dpy-19 family.