Disclosed is an enhancer for a viral promoter such as a promoter that can induce expression selectively and strongly in immunocompetent cells (e.g. lymphocytes) or blood cells. It is found unexpectedly that an intron has the above-mentioned enhancer activity. Thus it is found that an enhancer for a promoter which comprises an intron sequence for a major immediate early gene (MIE) of human herpes virus-6 (HHV-6) (particularly HHV-6B) or a fragment of the intron sequence has a potent promoter activity.
