RREB1 (Ras responsive element binding protein 1), also known as FINB, LZ321 or Zep-1, is a 1,687 amino acid protein that localizes to nuclear speckles and contains fifteen C2H2-type zinc fingers. Expressed in pancreas, heart, liver, placenta, lung, kidney and skeletal muscle, RREB1 functions as a transcription factor that binds to the Ras-responsive element (RRE) of target gene promotors, thereby inducing their transcription. In addition, RREB1 is thought to be involved in Raf/Ras-mediated cell differentiation, possibly by enhancing the expression of Calcitonin, a hormone that is produced by the thyroid gland and plays an essential role in maintaining blood calcium levels. Due to its involvement in cell differentiation events, RREB1 may be involved in Raf/Rasrelated tumor formation. Four isoforms of RREB1 exist due to alternative splicing.