Two studies published recently show that 13 of 16 children treated with gene therapy  treating diseases by correcting a patient's faulty genes - for severe combined immune deficiency, or SCID, have had their immune systems restored. The best treatment for the disease is a bone marrow transplant from an immunologically matched sibling. But when no matched donor is available, unmatched donors, such as parents, are recruited; these transplants are only around 70 percent successful. The success of gene therapy now rivals or betters that seen in these unmatched donor situations. In 2001, a child in the trial developed leukemia, thought to have been induced by a component in the modified virus, or vector, the researchers used to insert the correct gene into the boy's cells. Of the 30 children worldwide who have been treated with gene therapy for another form of SCID, marked by a deficiency in the enzyme adenosine deaminase (ADA), none has developed leukemia. Yet medical researchers maintain that gene therapy is still a better alternative than the conventional treatment for X-linked SCID in some children because 19 of the 20 children who have received gene therapy for X-linked SCID are still alive. When told these odds, all parents of children with X-linked SCID have opted for gene therapy

Question: Siblings are always immunologically matched.
Options:
A. true, B. false, C. uncertain
Answer: "C"