Somatic genome variation arising from copy-number variation (CNV) is thought to contribute to functional diversity in the human brain, but such variation has been difficult to measure. Here, the authors used two approaches that enable large-scale mapping of CNVs in single cells — namely, microarray analysis of multiple displacement amplification products and single-cell sequencing. They found that, in humans, the levels of CNVs are markedly higher in neurons than in non-neuronal cells such as fibroblasts.