Charcot-Marie-Tooth type 1A (CMT1A). It is the first delineated micro-duplication syndrome and is associ­ated with a micro-duplication of chromosome 17 (region 17p11.2) which contains PMP22, an important gene for peripheral nervemyelination. The main symptom of the syndrome is sensorimotor neuropathy [3,4]. Inheritance is autosomal dominant in pattern. Analysis of nerve biopsies suggests that the disorder is caused by increased gene dosage. Occasionally, CMT1A results from point muta­tions within the PMP22 gene. In cases with a duplication, onset of symptoms is usually in the first decade of life, while slowing of nerve conduction velocity is evident from the age of 2 years. The most common clinical phenotype is the CMT syndrome with distal muscle wasting and weakness, tendon areflexia, usually mild sensory loss, and foot deformity.

Smith-Magenis. Ninety percent of patients with Smith-Magenis syndrome have a micro-deletion of chro­mosome 17 (region 17q11.2), while a few cases (about 10%) have been described with micro-duplication of the same region. The main features of a micro-deletion are: mental retardation, congenital anomalies and behavioral problems, while a micro-duplication has a milder pheno­type [5,6].

Beckwith-Wiedemann. The cardinal features of this disorder, which is most often referred to as Beckwith-Wiedemann syndrome (BWS), are exophthalmus, macro­glossia and gigantism in the neonate. These patients are at risk of developing specific tumors. Beckwith-Wiedemann syndrome is caused by a mutation in the chromosome 11p15.5 region. Duplication in this region appears to be involved in the pathogenesis, and imprinting results in anomalous patterns of transmission. Most cases are spo­radic [7,8].

Down’s Syndrome with Normal Karyotypes. Down’s syndrome with a normal karyotype is caused by a micro-duplication of chromosome 21 (region 21q21-21q22.1) not seen even with high resolution conventional cyto­genetic techniques. The main features of this well recog­nized syndrome are: mental retardation, characteristic facies, heart defects, hypotonia, simian crease, etc. [9,10].

Figure 3. Prader-Willi region without a micro-dele­tion.