Gingival overgrowth is a frequent and adverse side-effect caused by certain 
immunosuppressant, anti-convulsant and calcium channel-blocking drugs. Although 
the precise mechanism is not yet known, the enhanced proliferation of gingival 
fibroblasts observed in this disease could be caused at least partly by the 
effects of the drugs on the cell cycle and cyclin expression in these cells. In 
the present study, flow cytometry analysis of the effects of the 
immunosuppressant drug cyclosporin A showed that it enhanced cell-cycle 
progression of gingival fibroblasts in vitro and also up-regulated the 
expression of cyclin B1. In addition, reverse transcriptase/polymerase chain 
reaction analysis of gingival overgrowth tissues showed markedly elevated 
transcription of the cyclin B1 gene. Thus, the increase in cell growth that 
occurs in drug-induced gingival overgrowth may be mediated by over-expression of 
cyclin B1.