It was discovered in 1937 by Karl Landsteiner and Alexander S. Wiener who, at the time, believed it to be a similar antigen found in rhesus monkey red blood cells. It was subsequently learned the human factor is not identical to the rhesus monkey factor; but by then, "Rhesus Group" and like terms were already in widespread, worldwide use. Thus, notwithstanding it is a misnomer, the term survives (e.g. rhesus blood group system, and the obsolete terms rhesus factor, rhesus positive, and rhesus negative - all three which actually refer specifically and only to the Rh D factor; and, thus, are misleading when unmodified).