To elucidate the functional and structure properties of I1R, several different epitope-specific antisera against IRAS have been raised in rabbits [23] . Because of IRAS splice variants or nonspecificity of these antisera, more sizes of IRAS (≈33, ≈85, ≈170 KDa) have been visualized in various tissue and cells, which limited their uses on western blot. IRAS was reported to target to the endosomes by a combined action of a PX domain and a coiled-coil region. The PX domain, consisting of 10-130 amino acids, was first identified from the sequence analysis of two SH3 domain-containing cytosolic components of NADPH oxidase, p47phox and p40phox [24] . Therefore, in the present study, we developed the newly monoclonal antibody against the N-terminal hIRAS region including the PX domain (10-120aa) . This development has great utility for immunoblotting, indirect immunofluorescent staining, immunoprecipitation, and flow cytometry. These monoclonal antibodies will provide powerful reagents for the further investigation of hIRAS protein functions.