A DNA microarray experiment. A microarray can be prepared from any known DNA sequence, from any source. Once the DNA is attached to a solid support, the microarray can be probed with other, fluorescently labeled nucleic acids. Here, mRNA samples are collected from frog cells at two different stages of development: single-cell stage (sample 1) and a later stage (sample 2). The cDNA probes are synthesized with nucleotides that fluoresce in different colors for each sample; a mixture of the cDNAs is used to probe the microarray. The probes anneal to spots containing complementary DNA; if the spot lights up, the corresponding gene is represented in the pool of mRNA used to produce the probes. Green spots represent mRNAs more abundant at the single-cell stage; red spots, sequences more abundant later in development. The yellow spots indicate approximately equal abundance at both stages.