In high-density oligonucleotide SNP arrays, hundreds of thousands of probes are arrayed on a small chip, allowing for many SNPs to be interrogated simultaneously.  Because SNP alleles only differ in one nucleotide and because it is difficult to achieve optimal hybridization conditions for all probes on the array, the target DNA has the potential to hybridize to mismatched probes.  This is addressed somewhat by using several redundant probes to interrogate each SNP.  Probes are designed to have the SNP site in several different locations as well as containing mismatches to the SNP allele. By comparing the differential amount of hybridization of the target DNA to each of these redundant probes, it is possible to determine specific homozygous and heterozygous alleles. Although oligonucleotide microarrays have a comparatively lower specificity and sensitivity, the scale of SNPs that can be interrogated is a major benefit.  The Affymetrix Human SNP 5.0 GeneChip performs a genome-wide assay that can genotype over 500,000 human SNPs (Affymetrix 2007)..