We describe the emerging role of Synovial Sarcoma X breakpoint 2 Interacting protein (SSX21P) in cancer and its still largely unknown function in human cells. In rodents, SSX21P has been shown to play a role in adherens junctions and cell adhesion, while in chickens SSX21P was identified by virtue of its regulation by the light cycle and circadian rhythms. In humans, SSX21P was identified through its interaction with the cancer-testis gene SSX2. However SSX21P is expressed in a range of normal and fetal tissues unlike SSX2. SSX21P containing constructs indicated that SSX21P could be expressed in the nucleus and cytoplasm of transfected human cells, however, SSX21P expression has been subsequently shown to peak on the surface of myeloid leukaemia cells during mitosis. Here we discuss the current knowledge of SSX21P function in several species and the growing evidence that SSX21P may be a suitable target for leukaemia immunotherapy. (C) 2007 Elsevier Inc. All rights reserved.