We speculated that abnormalities in the hippocampus in SIDS reflect proliferation and/or migration defects and are due to defective or deficient brainstem 5-HT innervation of the hippocampus, including innervation of the hippocampal Cajal Retzius cells that produce the reelin during development and regulate neuronal migration (24). This evidence concerns the gene RELN and sudden infant death syndrome.