As you are supporting the use of gene therapy which uses the idea that absent or damaged genes can be replaced or modified to help with diseases such as hemophilia or cystic fibrosis, the thesis statement must cover your reasons for being in support of a rather controversial therapy. List out your three main reasons for supporting gene therapy and see if you have enough to say or enough proof to use for each main idea. If you do, then put those three ideas in your thesis statement. For example, if you support gene therapy because even though it is controversial, the diseases researchers are trying to help often have few other alternatives, current research into a disease such as cystic fibrosis does not allow a long life for most of its sufferers and their situations are desperate, and gene therapy may lead to other ideas for researchers to pursue if allowed to continue researching with only volunteers who understand their risks. Whatever your big ideas are to support the use of gene therapy, be sure that you have enough to ensure that the reader clearly understands the reasons why you believe in gene therapy despite its rather controversial results so far.