STK39 (serine/threonine kinase 39, also known as SPAK) is a serine/threonine kinase that functions as a key effector in the WNK-SPAK/OSR1 signaling cascade 12. Its primary role involves regulating ion transport and blood pressure homeostasis by phosphorylating ion cotransporters downstream of WNK kinases 2. STK39 specifically phosphorylates Na-K-Cl cotransporters (NKCC1/2) and Na-Cl cotransporters (NCC) to promote their activation, while inhibiting K-Cl cotransporters (KCC2/3), thereby controlling renal sodium reabsorption in the distal nephron 13. This regulation is critical for mediating regulatory volume increase during hyperosmotic stress 41. Beyond ion transport, recent evidence reveals STK39's involvement in DNA damage response through phosphorylation of histone H2A.X downstream of ATM kinase, promoting homologous recombination repair 5. STK39 also participates in hepatocellular carcinoma progression via the PLK1/ERK signaling pathway 6. Clinically, STK39 genetic variants are associated with hypertension susceptibility, particularly in European and East Asian populations 7. Additionally, alternative STK39 transcripts with species-specific regulation have implications for antihypertensive drug development 8. These diverse functions establish STK39 as a potential therapeutic target for hypertension, cancer, and DNA-damage-related diseases.