A large number of cellular proteins bind ATP, frequently utilizing the free energy of ATP hydrolysis to drive specific biological reactions. Recently, a family of closely related ATP‐binding proteins has been identified, the members of which share considerable sequence identity. These proteins, from both prokaryotic and eukaryotic sources, presumably had a common evolutionary origin and include the product of the white locus of Drosophila, the P‐glycoprotein which confers multidrug resistance on mammalian tumours, and prokaryotic proteins associated with such diverse processes (...) as membrane transport, cell division, nodulation and DNA repair. A comparison of these various proteins provides valuable insights into the function and evolution of the multicomponent systems with which they are associated. (shrink)