Gene therapy is a promising treatment for a number of diseases where a "normal" gene carried by the vector is inserted into the genome, to replace an "abnormal" gene or supplement the expression of particular gene. Viral vectors are generally used but other nonviral methods of delivery are being developed. The treatment is still a risky option due to the viral vector used which can cause ill-effects, for example giving rise to insertional mutation that can result in cancer. However, there have been promising results.