Scientists' next step is to look for mutations in the DNA surrounding the SNPs that the patients have in common. The Affymetrix 10K Mapping array basically screens the entire human genome for 10,000 SNPs that scientists have discovered. On average, those SNPs are about 20,000 bases apart (an A, C, G or T molecule is called a "base"). In the example we've been using, scientists have found two SNPs, a G and a T, that are shared by people with a disorder like autism. That means out of the whole genome, scientists only have to look at the block of DNA containing those two SNPs to find the autism mutation. The next step would be to find out the exact order of the As, Cs, Gs and Ts on that block of DNA, which is called "sequencing." Researchers would sequence then that block of DNA from everyone in the study and then do a base-by-base comparison to try and find other mutations that people have in common, mutations that might be contributing to the autism.