Calcium and phosphate regulate PTH gene expression posttranscriptionally through the binding of trans-acting factors to a defined cis-acting instability element in the PTH mRNA 3'-untranslated region (UTR). We have previously defined AU-rich binding factor 1 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 supershift experiments with antibodies to Unr showed that Unr is part of the parathyroid RNA binding complex. Finally, because there is no parathyroid cell line, the functionality of Unr in regulating PTH mRNA levels was demonstrated in cotransfection experiments in heterologous human embryonic kidney 293 cells. Depletion of Unr by small interfering RNA decreased simian virus 40-driven PTH gene expression in human embryonic kidney 293 cells transiently cotransfected with the human PTH gene. Overexpression of Unr increased the rat full-length PTH mRNA levels but not a PTH mRNA lacking the terminal 60-nucleotide cis-acting protein binding region. Unr also stabilized a chimeric GH reporter mRNA that contained the rat PTH 63-nucleotide cis-acting element but not a truncated PTH element. Therefore, Unr binds to the PTH cis element and increases PTH mRNA levels, as does AU-rich binding factor 1. Our results suggest that Unr, together with the other proteins in the RNA binding complex, determines PTH mRNA stability.