Introductory comments on special section-genomic microduplications: When adding may equal subtracting.

The clinical implementation of array-based comparative genomic hybridization (aCGH) has allowed detection of copy number variations (CNVs) from megabases in size to those involving only a single exon. One major challenge that followed the clinical implementation of array CGH technology has been the interpretation of CNVs whose clinical significance can be elusive. The copy number gains resulting from genomic rearrangements are often more difficult to interpret than the copy number losses. Some of the CNV gains can be pathogenic, while others can be unrelated to disease since CNVs are often polymorphic in the normal population. The challenge faced by clinicians is how to differentiate between the disease causing CNVs and the nonpathogenic polymorphisms. Therefore, it is critical to systematically collect phenotypic information associated with CNVs and deposit it in searchable and publicly accessible databases. (c) 2010 Wiley-Liss, Inc.