Cyclin B is an important regulator of the G2 to M phase in the cell cycle.The silkworm genomic database showed that there are two Cyclin B genes in the silkworm (Bombyx mori),BmCyclin B and BmCyclin B3.Using silkworm ESTs data,the BmCyclin B3 (EU074796) gene was cloned.Its complete cDNA length was 1,665 bp and the ORF was 1,536 bp with six introns and seven exons.The BmCyclin B3 gene encoded 511 amino acids,and the predicted molecular weight was 57.8 kD with an isoelectric point of 9.18.It consisted of one protein damage box and two cyclin boxes.Cell cycle arrest in G2 or M phase in BmN-SWU1 cells could be induced by RNA interference-mediated reduction of BmCyclin B and BmCyclin B3 expression,thus inhibiting the cell proliferation.These results suggest that BmCyclin B and BmCyclin B3 are necessary for completing the cell cycle in silkworm cells.