Calcium and phosphate regulate PTH (PTH) gene expression post-transcriptionally through the binding of trans acting factors to a defined cis acting instability element in the PTH mRNA 3'-UTR. We have previously defined AU rich binding factor 1 (AUF1) as a PTH mRNA binding and stabilizing protein. We have now identified, by affinity chromatography, Upstream of N-ras (Unr) as another PTH mRNA 3'-UTR binding protein. Recombinant Unr bound the PTH 3'-UTR transcript and super-shift experiments with antibodies to Unr showed that Unr is part of the parathyroid (PT) RNA binding complex. Finally, since there is no PT cell line, the functionality of Unr in regulating PTH mRNA levels was demonstrated in co-transfection experiments in the heterologous human embryonic kidney (HEK)293 cells. Depletion of Unr by siRNA decreased SV40 driven PTH gene expression in HEK293 cells transiently co-transfected with the human PTH gene. Over-expression of Unr increased the rat full-length PTH mRNA levels but not a PTH mRNA lacking the terminal 60 nt cis acting protein binding region. Unr also stabilized a chimeric GH (GH) reporter mRNA that contained the rat PTH 63nt cis acting element but not a truncated PTH element. Therefore, Unr binds to the PTH cis element and increases PTH mRNA levles, as does AUF1. Our results suggest that Unr, together with the other proteins in the RNA binding complex, determines PTH mRNA stability.