SMS and Williams syndrome: In recent years, with the application of mechanistic insight, we have witnessed the defining of the Potocki-Lupski syndrome as the predicted reciprocal rearrangement of SMS, dup(22)(q11.2) as the reciprocal rearrangement of DG/VCFS, and dup(7)(q11.23) as the reciprocal rearrangement for Williams-Beuren syndrome deletion [50-55].