The copy number of a protein-making gene, DUF1220, which has been related in humans with microcephaly, may underly the extra size of our brains and therefore that of our rather unique intelligence.
Laura J. Dumas, DUF1220-Domain Copy Number Implicated in Human Brain-Size Pathology and Evolution. AJHG 2012. Pay per view (freely accessible after 6-month embargo) ··> LINK [doi:10.1016/j.ajhg.2012.07.016]
Humans have more than 270 copies of DUF1220 encoded in the genome, far more than other species. The closer a species is to humans, the more copies of DUF1220 show up. Chimpanzees have the next highest number, 125. Gorillas have 99, marmosets 30 and mice just one. "The one over-riding theme that we saw repeatedly was that the more copies of DUF1220 in the genome, the bigger the brain. And this held true whether we looked at different species or within the human population."
Source: Science Daily.