The diagnostic yield of karyotyping and FISH analysis in the general population of patients with unexplained mental retardation is approximately 10%. Consequently, a clinical diagnosis is lacking in the majority of these patients, as is the case for patients with other neurological and/or psychiatric disorders. The genomic microarray work on the human subtelomeres and the limited number of genomic regions involved in microdeletion syndromes demonstrates that microdeletions and microduplications, not visible by routine chromosome analysis, are a major cause of human malformation and mental retardation.

Recent developments in microarray technology, now allow a whole-genome analysis at an unprecedented resolution, 10–10,000 times higher than that of routine chromosome analysis by karyotyping. These advancements in genomic CNV analysis by microarray technologies have recently demonstrated that CNV contributes significantly to human genomic variation. Due to this variation, any two randomly chosen genomes differ by millions of bases containing thousands of protein-coding genes and other functionally important sequences. The impact of this genomic variation on normal phenotypic variation as well as disease susceptibility is largely unknown. Notwithstanding this uncertainty, microarray-based genome profiling is increasingly being used clinically to screen the genome for disease causing CNVs.