glia (DRGs) or their axons (Ernfors et al. 1994; Klein et al. 1994; Tessarollo et al. 1994; Fariñas et al. 1996; Liebl et al. 1997; Inoue et al. 2002; Levanon et al. 2002). NT-3 is expressed in the ventral spinal cord, in the developing limb buds, and in intrafusal bag fibers of muscle spindles later in development (Copray and Brouwer 1994; Fariñas et al. 1996; Tojo et al. 1996). When sensory ax