Citazione:

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Typically, the first transformation applied to expression data, referred to as normalization, adjusts the individual hybridization intensities to balance them appropriately so that meaningful biological comparisons can be made. There are a number of reasons why data must be normalized, including unequal quantities of starting RNA, differences in labeling or detection efficiencies between the fluorescent dyes used, and systematic biases in the measured expression levels. Conceptually, normalization is similar to adjusting expression levels measured by northern analysis or quantitative reverse transcription PCR (RT-PCR) relative to the expression of one or more reference genes whose levels are assumed to be constant between samples.

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