Retinoic acid-induced 1 (RAI1) is a dosage-sensitive gene, for which a 50% change in expression levels due to deletion/mutation or duplication of a single copy causes Smith–Magenis syndrome (SMS) [1,2,3] or Potocki–Lupski syndrome (PTLS) [4], respectively. The gene discussed is RAI1; the disease is Potocki-Lupski syndrome.