Although there are still many technical difficulties to be overcome, recent advances in the molecular and cellular biology of gene transfer have made it likely that gene therapy will soon start to play an increasing role in clinical practice. However. safety issues are raised from vector system. It is not clear whether it is safe to incorporate genes into nuclear DNA. Little is known about the antigenicity of gene product which the immune system is encountering. In this review, some safety-related topics are introduced and discussed.