Mutations in the LIS1 gene may result in severe abnormalities of brain cortical layering known as lissencephaly. Most lissencephaly-causing LIS1 mutations are deletions that encompass the entire gene, therefore the mechanism of the disease is regarded as haploinsufficiency. So far, 13 different intragenic mutations have been reported: one point mutation, H149R; deletion of exon 9, which results in deleted acids Delta301-334; deletion of exon 4, which results in deleted amino acids Delta40-64; 10 mutations resulting in truncated proteins and one predicted to result in extra amino acids. We studied the consequences of the point mutation, deletion mutation and one of the reported truncations. In order to study LIS1 structure function, we introduced an additional point mutation and other truncations in different regions of the protein. The consequences of these mutations to protein folding were studied by gel filtration, sucrose density gradient centrifugation and measuring resistance to trypsin cleavage. On the basis of our results, we suggest that all truncation mutations and lissencephaly-causing point mutations or internal deletion result in a reduction in the amount of correctly folded LIS1 protein.