Already, variations in the composition of about 500,000 DNA building blocks (SNPs or Single Nucleotide Polymorphisms), spread over the total length of the DNA and shown to correlate with particular physical and behavioral characteristics or susceptibility to diseases, are being analyzed routinely. Major errors in DNA composition that are responsible for about 3,000 of the 7,000 known genetic diseases can be visualized, and efforts are underway to identify the causes of the remaining 4,000.