The choice of receptor/ligand expression profiles. A, Density of ephrinA ligands in SC obtained from RNA hybridization . B, Density of EphB receptors in retina from RNA hybridization . C, density of RGC in retina . D, Density of EphA receptor per cell, obtained by dividing B by C. Vertical axes in A, B, and D are in arbitrary units. Dashed lines show the exponential approximations used in this study.