Migration of neurons and neural crest cells is of central importance to the development of nervous systems. In Caenorhabditis elegans, the QL neuroblast on the left migrates posteriorly, and QR on the right migrates anteriorly, despite similar lineages and birth positions with regard to the left-right axis. Initial migration is independent of a Wnt signal that controls later anteriorposterior Q descendant migration. Previous studies showed that the transmembrane proteins UNC-40/DCC and MIG-21, a novel thrombospondin type I repeat containing protein, act redundantly in left-side QL posterior migration. Here we show that the LAR receptor protein tyrosine phosphatase PTP-3 acts with MIG-21 in parallel to UNC-40 in QL posterior migration. We also show that in right-side QR, the UNC-40 and PTP-3/MIG-21 pathways mutually inhibit each other's role in posterior migration, allowing anterior QR migration. Finally, we present evidence that these proteins act autonomously in the Q neuroblasts. These studies indicate an inherent left-right asymmetry in the Q neuroblasts with regard to UNC-40, PTP-3, and MIG-21 function that results in posterior vs. anterior migration.C ELL migration is a fundamental event in the development of nervous systems. In the vertebrate central nervous system, neurons and neuroblasts migrate radially to populate distinct layers in the cerebellar and cerebral cortices, and neural crest cells migrate along distinct paths in the vertebrate embryo to give rise to the peripheral nervous system. The Q neuroblasts in Caenorhabditis elegans are a useful model to study the migration of neuroblasts and neurons in the anterior-posterior axis. The Q neuroblasts are a bilaterally symmetric pair of cells in the posteriorlateral region of the animal, with QR on the right side and QL on the left side (Sulston and Horvitz 1977). The Q neuroblasts are born in embryogenesis and are the sisters of the V5 hypodermal seam cells. By 5 hr after hatching, QR has migrated anteriorly and divided over the V4 seam cell, and QL has migrated posteriorly and divided over the V5 seam cell (Honigberg and Kenyon 2000;Chapman et al. 2008;Dyer et al. 2010). The resulting Q cell descendants then undergo a pattern of migration, division, and programmed cell death resulting in three neurons each (AQR, SDQR, and AVM on the right from QR; and PQR, SDQL, and PVM on the left from QL) (Sulston and Horvitz 1977;Chalfie and Sulston 1981). The QR descendant AQR migrates the longest distance to a region near the anterior deirid ganglion in the head, and the QL descendant PQR migrates the longest distance posteriorly to the phasmid ganglion in the tail (Sulston and Horvitz 1977;White et al. 1986;Chapman et al. 2008). The posterior migration of QL descendants requires the activity of the MAB-5/Hox transcription factor, expression of which is induced in QL descendants by an EGL-20/Wnt signal emanating from the posterior (Chalfie et al.