Kleefstra syndrome (KS; OMIM #610253), formerly known as the 9q subtelomeric deletion syndrome, is an autosomal dominant cause of intellectual disability (ID) characterized by hypotonia and facial dysmorphisms.(1,2) The cause of KS is attributed to haploinsufficiency of the euchromatin histone methyltransferase 1 (EHMT1) gene (OMIM *607001) located at chromosome 9q34.3 (i.e., distal long arm of chromosome 9), either by microdeletion or point mutation. EHMT1 encodes a histone H3 methyltransferase at position Lys-9 (H3K9).(1-3).