Zfp-42 (Rex1) is a known marker of pluripotency, and most notably found in undifferentiated embryonic stem cells. It is a 288-amino acid protein and has a calculated molecular mass of 32.3 kD. It has been identified in the testicles, expressed at the spermatocytes in the more inner layers of the testicles. Rex1 has been found to be critically important in maintaining proliferative state in mesenchymal stem cells(MSC), while simultaneously preventing differentiation.