several types of reporter genes are available for use in the construction of bioreporter organisms, and the signals they generate can usually be categorized as either colorimetric, fluorescent, luminescent, chemiluminescent or electrochemical. although each functions differently, their end product always remains the same – a measurable signal that is proportional to the concentration of the unique chemical or physical agent to which they have been exposed. in some instances, the signal only occurs when a secondary substrate is added to the bioassay ("luxab", luc, and aequorin). for other bioreporters, the signal must be activated by an external light source (gfp and umt), and for a select few bioreporters, the signal is completely self-induced, with no exogenous substrate or external activation being required ("luxcdabe"). the following sections outline in brief some of the reporter gene systems available and their existing applications.