“A triumph of science, a miracle of medicine” is how researcher Fyodor Urnov describes the gene-editing treatment that saved baby KJ Muldoon’s life. Now nine months old, KJ was born with a genetic condition called urea cycle disorder, which is fatal for many infants. Urnov was part of the research team supporting KJ's doctors, he tells us what gene-editing treatments could mean for others born with life-threatening conditions.