A gene coding for potato multicystatin (PMC), the crystalline inhibitor of cysteine proteases which is found in tubers, was isolated and characterized. The deduced polypeptide product of this genomic sequence is 757 amino acids long and has a molecular mass of 86,778 Da. It consists exclusively of eight closely related domains, with 53-89% identity of residues. Each repeated unit is homologous to the cystatin superfamily of cysteine protease inhibitors. To date, no other member of this family has been found to contain so many inhibitor domains in one polypeptide. Eight introns are proposed in the 3.5 kb of genomic DNA coding for PMC, one in each cystatin unit. There is a family of 4 to 6 such large genes in potato, while in pea and maize the homologues are much smaller, and probably code for single-domain cystatins. PMC transcripts are abundant in tubers, but scarce in undamaged leaves or stems of field-grown potatoes. The tuber messages are derived from at least four genes (including the cloned example). The pattern of gene expression, as well as the properties of the protein, suggest that PMC has a role in the plant's defense system.