Background: Oxidative stress responsive 1 (OSR1) is a STE20-related kinase involved in ion transport and fluid and ion homeostasis. OSR1 interacts with SPAK (STE20/SPS1-reated proline/alanine-rich kinase) and together are activated by the upstream kinase WNK1 [with no K (lysine) protein kinase-1]. Activation of OSR1 and SPAK by WNK1 results in the phosphorylation and activation of NKCC1 (Na+-K+-2Cl- co-transporter 1) for the control of cell volume and blood pressure.