With viral RNA extracted from purified beet necrotic yellow vein virus (BNYVV) isolated from Inner Mongolia, the first strand of cDNA encoding a 54kDa fragment of the 75kDa readthrough protein on the RNA2 was synthesized by reverse transcription. A double-strand cDNA fragment of 1.5 kb was obtained after 30 cycles of PCR amplification. The fragment was ligated into and mapped on pGEM-7Zf(+). The result of sequence analysis shows that the 54kDa readthrough domain is 1509 nucleotides (nt). Compared with F13 isolate, the fragment shares 94.97% identity in terms of nucleotides with 3 nt deletion and 96.42% identity of deduced amino acids.