In patients with MDS the 17p13.3 deletions are generally larger than those seen in patients with Isolated Lissencephaly Sequence (ILS) and deletions in the same area. A 2002 study narrowed the critical region that differentiates MDS from ILS down to 400kb. Patients with MDS tend to have classical lissencephaly at the most severe end of the spectrum, which is not normally seen in patients with ILS, although deletions or mutations of the LIS1 gene are responsible for lissencephaly in both cases