Cytochromes were initially described in 1884 by MacMunn as respiratory pigments (myohematin or histohematin).[4] In the 1920s, Keilin rediscovered these respiratory pigments and named them the cytochromes, or “cellular pigments”.[5] He classified these heme proteins on the basis of the position of their lowest energy absorption band in their reduced state, as cytochromes a (605 nm), b (≈565 nm), and c (550 nm).