(A) Two groups of neurons in the lateral motor column of the spinal cord target the developing limb. Although they leave the spinal cord together, their paths later diverge, with the neurons in the lateral group (green) innervating the dorsal (D) musculature and the neurons in the medial group (purple) innervating the ventral (V) musculature. Guidance molecules ensure that both sets of neurons reach the correct target. (B) In the absence of certain guidance molecules, one set of neurons might arrive at the wrong target. This defect is known as a guidance phenotype. (C) Hua et al. have discovered a pure stalling phenotype in which motor axons belonging to neurons in the lateral group set out on the correct path but stop growing, which results in the death of these neurons in the spinal cord.