The storage protein polymers in the endosperm, stabilised by disulphide bonds, determine a number of processing qualities of wheat dough. The enzyme protein disulphide isomerase (PDI), involved in the formation of disulphide bonds, is strongly suggested to play a role in the formation of wheat storage protein bodies. Reports of the rice mutant esp2 exhibiting aberrant storage protein deposition in conjunction with a lack of PDI expression provided strong indications of a direct role for PDI in storage protein deposition. The potential significance of wheat PDI prompted the present studies into exploring any orthology between wheat PDI genes and rice PDI and esp2 loci. By designing allele-specific (AS)-polymerase chain reaction (PCR) markers, two of the three wheat PDI genes could be genetically mapped to group 4 chromosomes and showed close association with GERMIN genes. Physical mapping led to localisation of wheat PDI genes to chromosomal "bins" on the proximal section of chromosome 4AL and distal sections of 4BS and 4DS. Identification of the putative PDI gene of rice and its comparison to the esp2 locus revealed that they were present at similar positions on the short arm of chromosome 11. Analysis of a large section of the PDI-containing section of rice chromosome 11S revealed a number of putative orthologues from The Institute for Genomic Research Triticum aestivum Gene Index database, of which five had been mapped, each localising to group 4 chromosomes, many in good agreement with our mapping results. The results strongly suggest a close linkage between the esp2 marker and the PDI gene of rice and an orthology between the PDI loci of rice and wheat and predict quantitative-trait loci involved in storage protein deposition at the PDI loci.