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One of the most notable trends in mammalian evolution is the massive increase in size of the cerebral cortex, especially in primates. Humans with autosomal recessive primary microcephaly (MCPH) show a small but otherwise grossly normal cerebral cortex associated with mild to moderate mental retardation 1, 2, 3, 4. Genes linked to this condition offer potential insights into the development and evolution of the cerebral cortex. Here we show that the most common cause of MCPH is homozygous mutation of ASPM, the human ortholog of the Drosophila melanogaster abnormal spindle gene (asp) 5, which is essential for normal mitotic spindle function in embryonic neuroblasts 6. The mouse gene Aspm is expressed specifically in the primary sites of prenatal cerebral cortical neurogenesis. Notably, the predicted ASPM proteins encode systematically larger numbers of repeated'IQ'domains between flies, mice and …