There has been an explosion of data describing newly recognized structural variants in the human genome. In the flurry of reporting, there has been no standard approach to collecting the data, assessing its quality or describing identified features. This risks becoming a rampant problem, in particular with respect to surveys of copy number variation and their application to disease studies. Here, we consider the challenges in characterizing and documenting genomic structural variants. From this, we derive recommendations for standards to be adopted, with the aim of ensuring the accurate presentation of this form of genetic variation to facilitate ongoing research.Structural variation in the genome refers to cytogenetically visible and (more commonly) submicroscopic variants, including deletions, insertions, duplications and large-scale copy number variants -collectively termed copy number variations (CNVs) -as well as inversions and translocations (Box 1)1-3. Genome scanning technologies are now commonplace in many laboratories, allowing new structural variation to be recognized from general population surveys4-12 or studies of diseases13-21. In fact, the Database of Genomic Variants4,22 (see list of databases in Table 1 This first round of observations came from several studies, each using a different technology platform and data processing algorithms, with different degrees of pre-and postexperimental standardization and validation. As a result, the data vary in quality and often have both high false-positive and false-negative rates. There is the very real possibility of the entire human genome soon being presented as 'structurally variant' in one form or another, based solely on studies of nondisease samples, which would be a distortion. It will be important for all future applications of structural variation information that the scope and detail of variants in the general population be accurately cataloged. In particular, medical genetics researchinvestigating structural variation profiles in individuals or clinical cohorts -will need a reliable foundation against which to interpret possible pathogenic findings in cytogenomic ( Fig. 1), linkage and genome-wide association studies21,23-25.The field of genomic structural variation, however, is on the cusp of change. Pioneering approaches, often fragmented or fraught with technical limitations, are being supplanted by new technologies that afford much higher resolution screening of the genome at lower cost. We anticipate that, in the next year, the quantity of structural variation data will increase by orders of magnitude owing to microarray-based experiments alone, not to mention the plethora soon to flow from clone-end6,26 or whole-genome sequencing experiments27-30. Many of these studies will survey nondisease samples for structural variation discovery to create control databases. Moreover, in little more than two years from the first description of global CNV distribution4,5, the field is poised to make structural variation analyses standard in the...