Fortunately, these methods are complementary of each other since for example while the spontaneous mutations are limited in target size, yet they yield functionally perturbed mutants of defined phenotype. Only recent references that have not been cited in these reviews are included here. In addition, two cartoons are used to visualize the mutated positions of cyt b . 1a indicates the amino acid residues of cyt b that affect inhibitor resistance or hypersensitivity, and Fig. 1b those that impair the function of the bc 1 complex.
Several observations derive from the compilation of cyt b mutants presented here.
The rather large number of available cyt b mutants affecting the Q o and Q i sites of the bc 1 complex is striking. Yet, it should be stressed that the distribution of the cyt b mutations is far from being random. From a complementary perspective, the data presented in Fig. 1 also indicate that no mutation affecting the binding of either the Q i or the Q o site inhibitors has been found until now either in the ab or the bc loops of cyt b , respectively. Further, the carboxyl terminal portion of cyt b extending from the middle of helix F to its last amino acid residue appears not involved in the formation of the quinone processing domains. In summary, crucial information has been obtained by studying the Inh R and the non functional mutations located in cyt b .