During early development of the vertebrate cerebral cortex,newborn neurons migrate along radial glia filaments from the ventricular zone into the cortical plate before arriving at the proper layer and forming appropriate synaptic connections.The molecular mechanisms underlying the guidance of radial migration remain largely unknown.Using in utero electroporation of small interference RNAs (siRNA),we found that knock down the expression of robo3,one of the receptors of the guidance factor Slit,lead to the failure of newborn neurons to enter the cortical plate and the failure of the midline crossing of axons of transfected neurons.Robo3 is unlikely to mediate an attractive signal from slit since ectopic neurons in deep cortical layers after silencing robo3 expression displayed normal radial ofientaton.