Microarray technologies provided a strong push forward for high-throughput genotyping, however they were originally introduced for the high-throughput analysis of gene expression. They were a natural evolution of classic DNA and RNA hybridization methods like Southern and Northern blotting through technological advances that made it possible to print thousands of features on a chip and hybridize them to the RNA (or cDNA) under investigation, allowing the quantification of gene transcripts across the genome and comparisons between healthy and diseased tissues. What could be considered the first expression array experiment was performed as early as 1987 by Kulesh et at (39) searching for genes responding to interferon. The first microarray experiment was reported by Schena et al (40) in 1995, followed by thousands of other reports.
