Abstract: Four fulllength cDNAs and two partial cDNAs, designated BpEXP1 to BpEXP5, and BpEXPL, encoding expansin homologues, were isolated from Betula platyphylla . The open reading frames of four fulllength cDNAs ranged from186 to 258 amino acids, with predicted molecular weights of 242 to 258 kDa and PI of 557 to 920.The deduced amino acid sequences of the five alphaexpansin include eight typical Cys residues in the N-termini of the proteins, four Trp residues near the C-termini and an HFD motif in the central region, which are all characteristic of expansins. Real time RT-PCR analyses in birch organs and tissues showed that BpEXP1 was specifically expressed in developing xylem exhibiting secondary growth, and was presumed that BpEXP1 was involved in the cell wall modification during the wood formation. By contrast, BpEXP3 was mostly expressed in developing capsule, and then was presumed that BpEXP3 was involved in the cell wall modification during the seed development. Finally, BpEXP1 and BpEXPL were chosen to construct the plant expression vector of pROKII-BpEXP1 and pROKII-BpEXPL.