We had previously identified a novel protein termed Rwdd1 whose expression in thymus is decreased in aged or oxidatively stressed mice. In the present study, we found that Rwdd1 expressed in both prokaryotic and eukaryotic cells showed a slower migration rate on SDS-PAGE gel. In addition, Rwdd1 was more sensitive to proteinase proteolysis. Furthermore, being a highly acidic protein which contains an RWD domain, Rwdd1 shared a high level of sequence similarity with Gir2, a member of the intrinsically unstructured protein (IUP). These findings suggest that Rwdd1 is a novel member of the IUP family.