Many readers have submitted this story about a breakthrough in our understanding of human DNA: in particular, how much variation can exist between peoples' genes and how genes are involved with certain diseases. "One person's DNA code can be as much as 10 percent different from another's, researchers said on Wednesday in a finding that questions the idea that everyone on Earth is 99.9 percent identical genetically. They said their new version of the human genetic map, or 'book of life,' fills in many missing pages and chapters to explain how genes are involved in common diseases. The Human Genome Project mapped the billions of letters that make up the human genetic code. Scientists later refined the map by looking for single variations called SNPs or single nucleotide polymorphisms. The CNV map gives researchers a different way to look for genes linked to diseases by identifying gains, losses, and alterations in the genome."