Deletions, duplications or rearrangements of genomic regions in the human genomes produce differences in gene copy numbers, referred to as copy number variations (CNV). Those variations account for a substantial portion of human genetic diversity, and in a few cases, have been associated with behavioral traits or increased susceptibility to disease. A new study describes a CNV map of the African-American genome, and compares frequencies of CNVs between African-American and white American/European populations.