Basophils are recognized as effectors of type 2 immune responses, producing IL-4 in response to various stimuli such as IL-3 and papain in addition to IgE. In this study, we have established a novel cell-based reporter system that can monitor the activation of the transcription factor NFAT using retroviral vectors. Using this system, we examined whether papain, which is known to induce IL-4 in an FcRγ-dependent manner, could be used to identify its receptor. We created a chimeric receptor in which the extracellular and transmembrane portions are CD8 and the cytoplasmic domain is FcRγ. This chimeric receptor was able to induce GFP in RBL by cross-linking with anti-CD8 antibody. Furthermore, we found that this chimeric receptor can function as a papain receptor, as GFP was upregulated by papain stimulation. On the other hand, statins were able to suppress the expression of GFP by IgE crosslinking. This reporter system can be used to transduce candidate receptors and examine their papain receptor activity using GFP expression as an indicator, and is therefore a useful system that can be used for expression-cloning of unknown papain receptors and for the study of various inhibitors.