In the Bone Marrow Transplant Unit at Great Ormond Street Hospital, doctors strive to save the lives of children with life-threatening congenital diseases. They must attempt treatments which have very uncertain chances of success and can even threaten the lives of the children they are trying to save.

Herb is just five years old and has a condition called Nemo Syndrome, an immune disorder which means his body cannot fight infections. Professor Paul Veys tells his parents that Herb is unlikely to reach the age of 10 unless he is treated - but the treatment itself will put his life at risk. Both the doctors and Herb's family face an acute dilemma: should they go ahead?

For Keano, who's nine, the dilemma is even more critical. Suffering from a condition called congenital neutropenia, he also needs a bone marrow transplant. But unlike Herb, there is no well-matched donor for him in any database in the world. The risk that he will not survive treatment is a daunting 30%. His mother must decide whether to risk going ahead to give her child the chance of a future.

Teigan, who's 10, suffers from a condition called SCID - Severe Combined Immunodeficiency. Teigan's health is too fragile to even attempt a bone marrow transplant, so the only option for her is a pioneering treatment - gene therapy. Professor Bobby Gaspar has been instrumental in developing gene therapy at Great Ormond Street over the last 15 years. It offers hope for children who were previously untreatable but the uncertainties are even greater than conventional treatment.

Filmed over a year, the programme tracks these three extraordinary cases, three children caught in the middle of medicine's struggle to push the boundaries of what is possible. Show less