Some blame vaccines, while others target mercury. But the truth is that nobody knows what causes autism or what exactly accounts for the recent rise in cases. According to new data released by the Federal Government in October, 1 in 100 American children is now affected by an autism spectrum disorder, up from the previous federal estimate of 1 in 150. The roots of the increase are still unclear, but researchers this year identified one possible genetic clue: variations on a region of chromosome 5, which appear to play a crucial role in about 15% of cases of autism. Working with the Autism Genetic Resource Exchange — a DNA database of more than 2,000 families affected by autism, and the largest genetic study of the disorder ever attempted — researchers zeroed in on variations in genes that code for proteins involved in forming connections in the brain. Differences in these particular genes are extremely common — present in more than half of healthy people — but they are even more common in people with autism, affecting 65%.