A variety of strategies have been used to obtain cDNA and genomic clones encoding ricin. Since their isolation these sequences have been manipulated to allow expression of A chain (19) and A chain mutants (15,20,34), B chain (14,21-23) and proricin (24). Utilizing structural information (35), precise changes have been introduced into both A and B chains with the aim of probing catalytic and sugar-binding residues, respectively. In the longer term, such manipulations, coupled with successful expression and purification schemes, will allow the delineation of functional residues and domains, ensuring that ricin remains the prototype plant toxin with which to study cellular intoxication and ribosome inactivation and to utilize in pharmaceutical product development.