As an example of MRV, copy number variation (CNV) has emerged as rare but powerful risk factor for neuropsychiatric disorders. CNVs are segments of the genome > 1000 bases where the number of copies of this segment is different from the expected number. Down’s syndrome is a very large example where three copies (instead of two) of chr21 is present. The chr22q11 hemideletion (one copy of chr22 from 17.3–20.3 million bases) is another example, and has been associated with multiple neuropsychiatric disorders. GWAS chips also contain many CNV probes, leading to increasing interest in this topic.
