Concurrent cryptic microdeletion and microduplication syndromes have recently started to reveal themselves with the advent of microarray technology. Analysis has shown that low-copy repeats (LCRs) have allowed chromosome regions throughout the genome to become hotspots for nonallelic homologous recombination to take place. Here, we report a case of a 7.5-year-old girl who manifests microcephaly, developmental delay, and mild dysmorphic features. Microarray analysis identified a microduplication in chromosome 17q21.31, which encompasses the CRHR1, MAPT, and KANSL1 genes, as well as a microdeletion in chromosome 7q31.33 that is localised within the GRM8 gene. To our knowledge this is one of only a few cases of 17q21.31 microduplication. The clinical phenotype of patients with this microduplication is milder than of those carrying the reciprocal microdeletions, and suggests that the lower incidence of the former compared to the latter may be due to underascertainment. 1. Introduction Since the advent of microarray technology considerable progress has been made in identifying small scale chromosome imbalances. The existence of colocalized microdeletion and microduplication syndrome sites has come to the fore in the recent years and a significant number of new microduplication syndromes have emerged such as 17p11.2 [1] and 22q11.21 [2]. These syndromes, like the corresponding microdeletion syndromes at these locations, appear to be driven by nonallelic homologous recombination (NAHR) involving low-copy repeats (LCRs or segmental duplications) [3–8]. LCRs are DNA fragments greater than 1？Kb in size, have 90% DNA sequence homology, and are thought to account for approximately 3–10% of the total genome. The MAPT gene located on chromosome 17q21.31 is flanked by LCRs and two extended haplotypes, designated H1 and H2, have been identified [9, 10]. The H2 haplotype is a 900？kb inversion polymorphism that has been reported as the likely ancestral state and which has a tendency to undergo recombination [11] leading to the 17q21.31 microdeletion syndrome. This syndrome has been well characterised and appears to be caused by haploinsufficiency of at least one gene, KANSL, within the deleted region [12, 13]. The more common H1 haplotype appears to be overrepresented in patients manifesting progressive supranuclear palsy [14]. Here, we report a 7.5-year-old girl with a 647？kb duplication involving interstitial chromosome region 17q21.31 as well as a 232？kb heterozygous interstitial deletion involving chromosome region 7q31.33. We review this case in conjunction with
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