Abstract: RNA interference(RNAi)technology has become the most effective approach for genetically engineering virus resistance in plants.Here,we constructed a hairpin RNA(hpRNA)-expressing RNAi vector targeting the replicase gene of a Chinese isolate of barley yellow dwarf virus(BYDV),BYDV-GAV.This construct was comprised of two tandem T-DNAs (twin T-DNAs),one of which contained the hygromyein resistance gene hpt as the selectable marker,and the other contained the expression cassette encoding hpRNA of the polymerase gene sequence of BYDV-GAV.This twin T-DNA construct should allow the production of selectable marker-free transgenic plants that are resistant to BYDV-GAV.Fig6,Ref21