Chemoattractants not only bring interneurons to the cortex, but also help to determine where in the cortex the interneurons will go. CXCL12 via an interaction with CXCR4 acts as a chemoattractant for interneurons whose role is not so much to guide interneurons to the cortex, but rather to make sure they end up in the appropriate location and cortical layer \cite{Daniel_2005,Tiveron_2006,Stumm2003}.  Interneurons will normally avoid entering the cortical plate for approximately 48 hours after reaching the cortex, during which time they spread out tangentially  \cite{López-Bendito2008}. However, distrubing the usual CXCL12 and CXCR4 signalling leads to early entry of the interneurons into the cortex and thus abnormal interneuron distribution \cite{López-Bendito2008}. CXCL12-CXCR4 are also important for getting interneurons to enter the migratory streams they will follow to the cortex \cite{Li2008}.
5. Neurotransmitters
    Neurotransmitters also appear to play a role in migration, especially GABA and dopamine. While migrating internruons express GABAA and GABAB receptors, reducing GABA levels leads to a decrease in the number of interneurons that cross into the cortical wall. In addition, migrating interneurons also have D1 and D2 dopamine receptors. Whereas D1 nulls show decreased ability to migrate, D2 knockoutss are show an increased propensity to migration. This suggests that D1 and D2 normally function to promote or inhibit, respectively, cortical interneuron migration.
6. Discussion 
Why are there two mechanisms of migration? Radial migration produces a one to one mapping between the originating location in the embryonic brain and the final destination in the cortex. Tangential migration does not depend on physical pathways established by RGCs and thus allows neurons from a given region to be distributed to multiple cortical regions in a one-to-many pattern. This may allow for an increase in the complexity of the cortical circuitry.