In the developing eye, has a role in axonal targeting of the R7 photoreceptor where it functions together with tutl. Probably mediates homotypic cell adhesion; the effect is inhibited by Lar. Interacts with tutl. In the visual system, expressed in lamina and medulla (at protein level). During the mid-pupal stage, strongly expressed in the lamina, medulla and inner optic lobe of the developing eye. Specifically detected in R7 and R8 axonal terminals in the medulla. In the eye, axonal targeting of the R1-R6, R7 and R8 cells appears to be normal. Double knockouts of bdl and tutl rescue the R7 axonal tiling defect of tutl mutants. Double knockouts of bdl and Lar partly rescue the R7 axonal targeting defect of Lar mutants. Belongs to the immunoglobulin superfamily. Truncated N-terminus.