The high molecular weight glutenin subunits (HMW-GS) are key factors of the breadmaking quality of common wheat flour. In the present study, one unexpressed 1By gene from Triticum durum cultivar youmangbingmai was cloned and characterized. The results indicated that the silenced 1By gene in youmangbingmai showed the highest homology to that of previously published 1By8, suggesting that the unexpressed 1By may have originated from 1By8. However, compared to expressed 1By8, the silenced 1By in youmangbingmai had a deletion of 247bp at 5' segment. The deletion resulted in the intact of DNA sequences responsible for signal peptide and N- terminal domains. The deletion also resulted in frameshift mutation in C-terminal domains and central region. Therefore, 1By gene in youmangbingmai had no the typical structure as did normal HMW-GS genes and thus this gene can be not translated into normal HWM-GS. This may be the molecular mechanism of silence for 1By gene in youmangbingmai. This molecular mechanism is different from those previously reported. Previous studies indicated that the insertion of transposon elements or the presence of premature stop codon is the molecular mechanisms responsible for silence of HWM-GS genes. Moreover, this deletion of 247 nucleotides in YMBM produced a new reading frame. This reading frame may produce a new protein, which is different from HWM-GS.