The currently accepted model is that Slit, produced by midline cells, repels axons from the midline via Robo receptors. Ipsilaterally projecting (non-crossing) axons always have Robo receptors on their surface, while commissural axons have very little or no Robo on their surface, allowing them to be attracted to the midline by Netrins and, probably, other as-yet unidentified cues. After crossing, however, Robo receptors are strongly upregulated on the axon, which allows Robo-mediated repulsion to overcome attraction to the midline. This dynamic regulation of Robo is at least in part accomplished by a molecule called Comm (short for Commissureless), which prevents Robo from reaching the cell surface and targeting it for destruction.