Gene therapy can be used to treat some genetic disorders. An appropriate vector is chosen to carry the normal allele into the target cell. Three types of vectors commonly chosen are naked DNA, viruses and liposomes.

1. (a)A trial of gene therapy to treat cystic fibrosis used a viral vector. The viral vector caused a primary immune response with the production of memory cells. Explain why the production of memory cells prevents the gene therapy from working in long-term chronic conditions such as cystic fibrosis.[3]
2. (b)With reference to the three types of vectors that are commonly used, discuss the challenges in choosing appropriate vectors for use in gene therapy. Do not include problems associated with an immune response in your answer.[4]
3. (c)(i)A trial was carried out to find a new vector for use in gene therapy. The new vector was made from red blood cells taken from the person with the genetic disorder. The cells had most of their cytoplasmic content removed and were then broken up to make small spherical vectors. Most of these vectors lacked the ability to bind to receptors on the target cells. To solve this problem, genetically engineered stem cells taken from the person were used to form red blood cells. These red blood cells had membrane proteins that were complementary to the target cell receptors. The vectors that were produced were well-tolerated by the immune system. Explain why the vectors were well-tolerated by the immune system.[2]
4. (c)(ii)Suggest why it is not possible to produce genetically engineered red blood cells, except by using genetically engineered stem cells.[1]