FXYD1 (phospholemman) is a single-transmembrane protein that serves as a key regulator of Na+,K+-ATPase activity across multiple tissues. The protein associates with and modulates Na+,K+-ATPase function through phosphorylation-dependent mechanisms, where unphosphorylated FXYD1 inhibits the pump while phosphorylation at Ser-63 or Ser-68 relieves this inhibition and can even stimulate activity beyond baseline levels 1. Mechanistically, FXYD1 slows the conformational transition E2(2K)ATP → E1(3Na)ATP and increases sodium binding affinity, with its cytoplasmic helix docking between αN and αP domains in the E2 conformation 1. FXYD1 is abundantly expressed in metabolically active tissues including heart, kidney, skeletal muscle, and gastrointestinal tract 2. Disease relevance includes downregulation in Hirschsprung's disease affecting colonic function 3, upregulation in recurrent miscarriage where it impairs decidualization by reducing Na+,K+-ATPase activity 4, and altered expression in neurodegenerative diseases contributing to extracellular potassium dysregulation 5. Clinical significance extends to exercise physiology, where FXYD1 expression increases with blood flow restriction training through oxidative stress and AMPK signaling pathways 6, highlighting its role in cellular adaptation to metabolic stress.