S I kKr

provides a measure of relative abundance of the RNA from one sample with respect to the RNA from another sample, and hence the relative expression level of the corresponding gene in the samples. On the other hand, it has been shown that not all cDNA sample probe sequences label symmetrically with Cy3 and Cy5 [106] and paired dye-swapping experiments are performed (i.e., if the first hybridization is control (Cy3) versus test (Cy5), then switch dyes for the second hybridization). If the labeling were symmetrical, then the plot of versus for every probe would be line of slope one

* 1 * " ' HybrU.tioi. I \ c*s / ||ybrldls¬ęlkon 3

through the origin.

In summary, each robotically spotted microarray experiment has its own built-in control, and results are given in terms of fold change or difference from a control situation. However, the measurement of absolute quantities and variations are more challenging in this technology.

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