The low-molecular weight (LMW) glutenin subunits are major determinants of the viscoelasticity of durum wheat gluten, and therefore of its technological quality, with both quantitative effects and qualitative effects. We have modified the LMW glutenin subunit composition of the durum wheat cultivar Ofanto by expression of a transgene encoding a B-type LMW glutenin subunit and have carried out detailed analyses of two independent transformed lines in order to assess the effect of the transgene on the size distribution of the glutenin polymers and on their functional properties. In one line the expression of the transgene led to an increase in the amount of large glutenin polymers resulting in stronger and more stable dough. In the second line, however, the expression of the transgenic subunit was accompanied by decreased expression of endogenous LMW subunits with consequent detrimental effects on glutenin polymers and dough viscoelasticity. These results demonstrate that the LMW glutenin subunits contribute to the functional properties of wheat by influencing the amount and the distribution of glutenin polymers and indicate that either plant breeding or GM technology can be used to 'fine tune' the properties of durum wheat for different end uses by manipulating the amount and structures of individual LMW subunit proteins.