Obtaining efficient levels of folding has long been a problem in recombinant protein production. Because each protein has its particular requirements for efficient folding, purification protocols have to be designed on a case by case basis. Using a complex of GroEL chaperone, the DsbA foldase, and protein disulfide isomerase immobilized on agarose gel, Fersht and colleagues have provided a potential solution. They have refolded scorpion toxin Cn5, a protein traditionally very difficult to refold, even under optimal conditions (see pp. 136 and 187), to recover an 87% yield of Cn5 with 100% biological activity.