TUC (TOAD [Turned On After Division]/ Ulip/CRMP) is a family of proteins that have emerged as strong candidates for a role in neuronal growth cone signaling. The TUC-4 family members reach their highest expression levels in early post-mitotic neurons during their peak periods of axonal growth. TUC-4 is not expressed in neural progenitors or adult neurons (with minor exception). Data suggests that TUC-4 plays a role in axonal outgrowth, pathfinding and possibly in the process that permits growing axons to choose proper routes and targets (4). The TUC-4 protein is expressed primarily within the cytoplasm of postmitotic neurons as they begin their migration out of the ventricular zone into the developing cortical plate (2). TUC-4 is expressed in neuronal cell bodies, neurites, and along the most distal regions of neuronal growth cones, including the lamellipodia and filopodia (3). Expression of TUC-4 is observed by day E12 in rats coincident with the expression of the early neuronal marker class III beta-tubulin (4). TUC-4 expression is rarely observed in neuronal cells expressing the mitotic marker PCNA (2). The expression of TUC-4 has been shown to be upregulated in response to NGF in rats (3) and in response to retinoic acid in a human neuroblastoma cell line (SMS-KCNR).