Urea plays an important role in the urinary concentrating mechanisms. In the past decade, significant progress has been made in understanding urea reabsorption and recycling in kidney including the cloning of the two facilitated urea transporter genes, UT-A and UT-B. However, strong evidence from physiological studies demonstrated that the existence of a sodium- dependent active transporter in mammalian kidney, which can uphill move urea in some nephron segments. Though the active urea transporter was proposed over 40 years ago, none has yet been characterized. The overall objective of this project is to clone this novel active urea transporter from kidney inner medulla (IM) by using two independent approaches: 1) modified two-tester suppression subtraction hybridization (ttSSH) and 2) expression screening using a urea uptake defective yeast strain. After having successfully identified the novel urea transporter, we will investigate the nature of the active urea transporter. We hope the two approaches we apply here will lead us to discover the new genes. We believe the success of this project will fill in the gap with the molecular evidence how urea is actively transported in kidney and will extend our understanding on the urinary concentrating mechanism and water homeostasis. PUBLIC HEALTH RELEVANCE Strong physiological evidence demonstrated the existence of active urea transport activity in mammalian kidney over 40 years. The overall objective of this project is to clone the novel active urea transporter from kidney IM by using two independent approaches. The success of this project will extend our understanding on the urinary concentrating mechanism and water homeostasis.