Transmembrane protein required for the axon guidance of a subset of ventral nerve cord-associated interneurons and motor neurons. May function with the netrin receptor unc-40 in axon guidance. Interacts with unc-40 (via cytoplasmic domain). Expressed in the AVG pioneer midline neuron and in several nerve ring neurons that extend projecting axons into the right ventral nerve cord. During the 1.5 fold embryo stage, detected in the AVG pioneer neuron and in the developing ventral nerve cord. At larval stage L1, detected in the AVG, CEPDL/R, RMDVL/R, RIVL/R, AVAL/R, RMDL/R, and RMDDL/R neurons. At larval stage L4, expression continues in the AVG neuron and a subset of nerve ring neurons that extend axons into the right ventral nerve cord. The cytochrome b5 heme-binding domain lacks the conserved iron-binding His residues at positions 84 and 106. Axon guidance defects of nerve ring-associated interneurons that extend their axons posteriorly into the right ventral nerve cord as well as defects in the axon guidance of D-type motor neurons that project their axons longitudinally along the ventral nerve cord. No axon guidance defects are observed for the AVG midline neuron. Belongs to the cytochrome b5 family. MAPR subfamily.