ROBO4 and lung cancer: Both Slit2-WT and Slit2-ΔE15 inhibit lung cancer cell invasion, but Robo4 is essential for Slit2-WT-mediated invasion inhibition in CL1-5 cells, while neither Robo1 nor Robo4 is required for Slit2-ΔE15-mediated invasion inhibition.