1. An isolated caltubin protein which exhibits axon regenerating activity, said protein comprising the amino acid sequence of SEQ ID NO: 18, or a functionally equivalent N-terminal fragment of SEQ ID NO: 18 comprising at least amino acid residues 80-112 of SEQ ID NO: 18, or a functionally equivalent variant of said protein or fragment that exhibits at least about 90% homology thereto, wherein the protein, fragment, or variant is linked to a cell-penetrating peptide.