Inability of known chemoattractants to mimic the attractive activity in the OB. A, Chemoattraction of E12 commissural axons by netrin-1 expressed from a stable HEK cell line. dSP, Dorsal spinal cord. B, Chemorepulsion of SVZa cells by netrin-1 from the same HEK cell line as those used in A. C, Chemoattraction of E15 upper rhombic lip (URL) by SDF-1 expressed from a stable HEK cell line. D, Absence of guidance activity on SVZa neurons by SDF-1 from the same cell line as that in C. E, Chemorepulsion of E14 dorsal root ganglion (DRG) axon by Sema 3A transiently expressed from an HEK cell line. F, Absence of guidance activity on SVZa neurons by Sema 3A transiently expressed from an HEK cell line. Scale bar, 100 μm.