Corticospinal neurons project to various subcortical targets including the basilar pons via axon collateral branches. Previously, we showed that the pons produces diffusible chemoattractant to promote branch initiation. However, the molecular mechanism underlying axon collateralization is still unclear. To identify molecules that are essential for axon collateralization, we performed knockdown experiments for various receptors expressed in the corticospinal neurons. We found that knockdown of one of the receptor protein tyrosine phosphatases (RPTPs), tentatively called RPTP1, of the corticospinal neurons at E12.5 significantly suppressed the formation of axon collaterals to the pons. We purified proteins interacting with extracellular region of RPTP1 from the brain lysates by affinity chromatography in order to seek for ligands for RPTP1, and we identified several secreted proteins, proteoglycans and transmembrane proteins by mass spectrometry. We found that some candidate molecules were directly bound to RPTP1, and that the formation of axon collaterals was suppressed by knockdown of some of these molecules.