Getting rid of carbon dioxide In mammals, red blood cells deliver oxygen to tissues and remove carbon dioxide. Key to this essential process is a membrane protein called anion exchanger 1 (AE1) which transports bicarbonate (formed from carbon dioxide) out of red blood cells in exchange for chloride. This decreases the pH inside the blood cells, so that oxygen is released from hemoglobin and can diffuse into tissues. Arakawa et al. report the crystal structure of the transmembrane anion exchanger domain of AE1, which includes 14 transmembrane helices. The structure provides a basis for understanding the effects of mutations that lead to red blood cell diseases and also gives insight into the mechanism of ion transport. Science , this issue p. 680