These hybridizations are performed to study the expression of the gene. RNA is typically isolated from different tissues and from different developmental stages of species. After electrophoresis, the RNA is transferred to the membrane and probed. If, for example, the probe hybridizes only to RNA from heart tissue after the individual reaches adult age, it can be concluded that the gene is only expressed in the adult heart. In general, genes can be expressed constitutively, temporally or spatially.
Constitutive expression implies that the gene is expressed at all times.
If a gene is only expressed during a specific time in development it is said to exhibit temporal expression . Combinations of expression patterns are also possible. For example, a gene that is always expressed in a leaf tissue is exhibiting constitutive, spatial expression.