M. Schena is the first author of a 1995 demonstration of parallel gene expression measurement using a DNA microarray. The team robotically printed Complementary DNA on glass, turning the positions of small spots into addresses for different genes. Simultaneous measurements with two colors of fluorescence compared expression of 45 genes from Arabidopsis thaliana. The small array permitted a reaction volume of two microlitres and probes prepared from two micrograms of cellular messenger RNA.[ref]
The useful change was from measuring one gene at a time to comparing many genes within the same experiment. As a simplified numerical illustration, comparable, corrected fluorescence signals of 400 and 100 units at one spot would give a ratio of four. That ratio concerns the assay's RNA-derived signal under its measurement assumptions; it is not automatically a fourfold change in the amount or activity of the corresponding protein. The original experiment was a 45-gene demonstration, so its scope should not be confused with a measurement of an entire human genome.
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