To what extent are de novo DNA rearrangements in the human genome responsible for sporadic human traits including birth defects? How many human Mendelian and complex traits are due to structural changes and/or gene copy number variation (CNV)? What are the molecular mechanisms for human genomic rearrangements? The answers to these questions will impact both prenatal and postnatal genetic diagnostics, as well as patient management and therapeutics. Fifteen years ago, it became evident that genomic rearrangements and gene dosage effects, rather than the classical model of coding region DNA sequence alterations, could be responsible for a common, autosomal dominant, adult-onset neurodegenerative trait-Charcot-Marie-Tooth neuropathy type 1A (CMT1A). With the identification of the CMT1A duplication and its reciprocal deletion causing hereditary neuropathy with liability to pressure palsies (HNPP), the demonstration that PMP22 copy-number variation (CNV) could cause inherited disease in the absence of coding-sequence alterations, was initially hard to fathom. How could such subtle changes-three copies of the normal “wild-type” PMP22 gene rather than the usual two-underlie neurologic disease?Central to our understanding of human biology, evolution, and disease is an answer to the following questions: What is the frequency of de novo structural genomic changes in the human genome? and What are the molecular mechanisms for genomic rearrangements?