Chromosome 21 is the smallest chromosome in the human body and accounts for 1.5% of the total DNA in our cells. It contains 200-300 genes. In Down Syndrome, the extra genetic material from the third chromosome 21 results in over expression, or under expression, of a portion of the genes located on this chromosome. Research is still underway to determine how the alteredaltealalterfda expression of certain proteins causes the specific traits of a Down Syndrome person, but what scientists have found so far has helped explain several of them. (Science Direct, Gene Expression Profiling in the Adult Down Syndrome Brain)