RHAG (Rh-associated glycoprotein) is a transmembrane protein essential for erythrocyte membrane stability and function. As a core component of the ankyrin-1 complex, RHAG tethers the spectrin-actin cytoskeleton to the erythrocyte membrane and facilitates clustering of membrane proteins 1. RHAG forms a heterotrimer with RHCE proteins that functions as the primary ammonium transporter in red blood cells, mediating NH4+ and methylammonium transport in both neutral and ionic forms 23. The NH4+ transport is electrogenic and represents the dominant ammonium conductance pathway 3. RHAG may also function as a CO2 channel, though evidence suggests AQP1 plays a more prominent role in CO2 transport 3. Additionally, RHAG regulates RhD membrane expression and is essential for proper Rhesus blood group antigen presentation 4. Clinically, RHAG mutations cause regulator-type Rh-null hemolytic anemia and overhydrated hereditary stomatocytosis, manifesting as morphological and functional erythrocyte abnormalities. Recent findings identify low-prevalence RHAG blood group antigens (RHAG5-7) resulting from rare missense mutations, some implicated in hemolytic disease of the fetus and newborn 5, highlighting RHAG's critical role in transfusion medicine.