Expression profiling is a logical next step after sequencing the genome, the sequence tells us what the cell could possibly do, while the expression profile tells us what it is actually doing at a point in time. Expression profiling experiments often involve measuring the relative amount of mRNA expressed in two or more experimental conditions. This is because altered levels of a specific sequence of mRNA suggest a changed need for the protein coded by the mRNA, perhaps indicating a homeostatic response or a pathological condition.