Membrane proteins: do they carry the gas ammonia or the ammonium ion in their luggage? And is transport active or passive? Biochemists have long speculated on the mechanistic details of the ammonium transport family of proteins (Amt), which include the Rhesus protein factors, known as the mammalian blood group system. In mammals, Rhesus proteins regulate acid and ion balance in kidney and liver cells. New research sheds new light on this issue.