1. C. SCID-X1 (7C deficiency) is an optimal model for gene therapy because there is little 7C gene transcription regulation; 7C expression is ubiquitous and constitutive among different hematopoietic lineages; and 7C exerts no autonomous function.

2. D. The vector should have no size limit to the genetic material it can deliver. The coding sequence of a therapeutic gene can vary from several hundred base pairs to more than 10,000 base pairs. In addition, the requirement for appropriate regulatory sequences may be required for efficient transduction and expression of the therapeutic DNA. The ability to produce a high titer on a commercial scale is essential to carry out large-scale tests. It is necessary to be able to transfer genes in nonreplicating and replicating cells. It is also important to optimize delivery to target cells and minimize delivery to normal cells.

3. A. OTC is a metabolic enzyme required to break down ammonia. Total lack of this enzyme leads to death shortly after birth owing to a buildup of ammonia. The partial presence of OTC also leads to accumulation of ammonia, which can be controlled by drugs and dietary intake. The genetic cause of this disease, its morbidity, and the need for rapid production of OTC by adenoviral vectors may extend the life span of OTC-deficient newborns to allow for drug treatment and dietary manipulation. Jesse Gelsinger, the 18-year-old patient who was the first patient to die on a phase I gene therapy trial, had OTC deficiency.

4. C. Systemic administration of adenoviral vectors has not been used in the treatment of hemophilia because of the transient gene expression and im-munogenic consequences of adenoviral delivery. All of the other approaches are under investigation or have been published in the literature on treatment of hemophilia.

5. B. dl1520 (Onyx-O15) was the first adenovirus developed with deletion of a gene encoding a p53-in-hibitory protein, E1B-55kD, theoretically making it selective for tumor cells that have lost p53 function. Controversial data demonstrate that the mechanism of selectivity is more complex than originally thought. In addition, clinical results have demonstrated responses in patients whose tumors did not have mutant p53.

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