Using DNA microarrays to measure gene expression changes in a tumor. mRNA is isolated from two samples, tumor cells and a control sample. From these transcripts, cDNA is prepared in the presence of a fluorescent nucleotide, with a red label for the tumor sample and a green label for the control sample. The cDNA strands are separated, hybridized to the microarray, and the unbound DNA is washed away. Spots that are red indicate genes which are expressed more highly in the tumor, while the green spots indicate reduced expression relative to control. Spots that are black or yellow indicate comparable expression at either low or high levels, respectively.

[Information from D. L. Nelson and M. M. Cox, Lehninger Principles of Biochemistry 6th ed. (W. H. Freeman and Company, 2013)]