"The same genes are expressed in the same brain regions in human, chimp and gorilla, but in different amounts. Thousands of differences like these affect brain development and function, and help explain why the human brain is larger and smarter."

http://www.amnh.org/exhibitions/permanent-exhibitions/human-origins-and-cultural-halls/anne-and-bernard-spitzer-hall-of-human-origins/understanding-our-past/dna-comparing-humans-and-chimps/

DNA: Comparing Humans and Chimps