Statistics and Data Analysis for Microarrays Using R and Bioconductor, 2nd Edition

The cell and its basic mechanisms 35

general, there is a quantitative correspondence b etween the amount of mRNA

produced by the enzyme reading the gene and the amount of protein pr oduced.

Therefore, the amount of mRNA produced from various genes is usually di-

rectly proportio nal to the amount of protein produced from that mRNA, i.e.

to the expression level of that gene. This is the main assumption at the basis

of most experiments that try to characterize the gene’s expression levels using

DNA micr oarrays. Nevertheless, one should keep in mind that the measured

levels of mRNA do not always map to proportiona l levels of protein, and even

if they did, not all those proteins may be in an active form, etc. A number

of other techniques are available ...