... researchers isolated about 100 neurons from three people posthumously. The scientists took a high-level view of the entire genome -- looking for large deletions and duplications of DNA called copy number variations or CNVs -- and found that as many as 41 percent of neurons had at least one unique, massive CNV that arose spontaneously, meaning it wasn't passed down from a parent. The CNVs are spread throughout the genome, the team found.

Edit: see the paper for more precise statements.