The genetic factors associated with variability in CYP2C9 induction have not been elucidated. The hypothesis underlying this study is that following 7 days treatment with rifampicin, a known CYP450 inducer, the genetic variability in regulatory elements controlling CYP2C9 expression will be associated with the magnitude of change in INR response to warfarin and in CYP2C9-mediated warfarin metabolism.