Validation of the animal model is important to establish whether the reporter activity correlates with receptor activation through the targeted pathways. Nonetheless, the ERE-Luc mouse demonstrated the existence of unliganded activation of ERs [30] . Thus, animal-based reporter assays may provide new insights into the physiology of target pathways. Indeed, the use of the reporter mice revealed differential mechanisms of ER activation in reproductive and non-reproductive tissues [31] . These results indicate that reporter mice are useful for biological research. Precise toxicology studies may also be performed by crossing another mouse model, such as the humanized liver metabolism model, with reporter mice [32] . Such studies might require that the high costs of compound provision are met, and that animal welfare conditions are appropriate. Table 1 summarizes the properties of reporter proteins. 