An example is the use of genetically engineered mouse models. It is possible to engineer a mouse so that it is incapable of expressing a gene, potentially revealing how other proteins in the organism are affected by such a loss. Additional constraints on the utility of such models, beyond the appropriateness of the model itself, derive from the size of the organism, the reproductive turnover time, and similar factors. Other proteins serve as cellular receptors that mediate specific cellular processes. Once associated with disease, these receptors can serve as important targets for development of new protein , antibody, and small-molecule drugs.
Abnormal genes can also serve as diagnostic markers for associated diseases.
Though we speak of a particular drug s target in the singular, no gene functions in an isolated fashion. Rather, all genes, and the proteins they encode, are parts of larger systems, interacting with each other in complicated ways. These systems are known as pathways, and as the expression of a particular gene is regulated by the body to occur at specific times and in specific locations, so the expression of entire pathways of genes is likewise regulated. The aim of expression profiling is to use the patterns of gene expression as a clue for understanding the underlying pathways. Most expression analyses rely on the mRNA transcripts that are preludes to the synthesis of proteins as the signal that expression has been turned on.