Transcript for 2 people try revolutionary drug to keep from going blind: Part 1
Other than the sun, I've never seen a star, ever. Reporter: Imagine a world without starlight. I've been told all my life, my vision is going to go. It's happening. And I get really scared. Reporter: A world without I couldn't really see the castle because it was so dark. I immediately thought that something was wrong with her eyes. Knowing that both my husband and I gave her a mutated gene. Reporter: Tonight, two incredible journeys. Jesus, please fix my eyes. Amen. Reporter: One revolutionary new drug. This was an eureka moment. Just knock your socks off. Reporter: And a race against time to stop the world from going dark. If I had the chance to save her vision, sign me up. I can't believe it's finally happening. Happy birthday to you Reporter: 6-year-old Monroe Lee dreams like many other girls, in visions of pink and princesses, but Monroe isn't like most little girls, because her dreams just don't match her reality. I couldn't really see in the dark. Reporter: Does she explain this to her friends? What does she say? Monroe can't see in the dark. Reporter: She suffers from a rare genetic disease that means she will eventually go blind, for a child who loves horse racing. He won! He won, yes! Reporter: And Disney. Everything Disney. That means never experiencing the glowing wonder of Cinderella's castle at night. I couldn't castle because it was so dark. Hi, pretty girl. Reporter: Monroe's mom says she noticed immediately that something was wrong when Monroe was just a baby. And it got worse. Monroe kept missing key milestones. So at 18 months she's still not walking. She did this scoot jump like crawl. Reporter: It wasn't until she was 3 years old that three learned her diagnosis, a genetic mutation inherited from her parents. Only about 2,000 people in the whole country have it. It was scary. It was scary. I mean, knowing that both my husband and I gave her a mutated gene. Reporter: But there's an extremely rare condition. We both blamed ourselves for a long time. Reporter: While her family has given her the best childhood possible, they know time is precious. If Monroe wanted to do it, we did it, because tomorrow wasn't promised. I didn't know if I'd wake up one day and she wouldn't be able to see at all. Look at mama. Monroe, she can't see. Reporter: Heather is going through the same thing. People always say they can't tell I'm blind. And I think that comes down to me being used to it and not letting me stop me from anything. Reporter: For the 25-year-old, life is one big blur. As a child, she was diagnosed with the same genetic disease that Monroe has. It goes by lca. I've been told all my life my vision's going to go. It's happening, and I get really scared. I try not to think about it. Reporter: The main issue with lca is light sensitivity. Heather can't see anything at night, a tough thing for someone who wants to be an astro physicist and study the stars. You have no idea what they look like. I've only seen them in pictures, that's it. Reporter: Even on the brightest day, Heather sees just a fraction of what other people do, using her guide dog Luna to get around. I can see the trees where they're at as blobs, things that are tall, and I can tell them different from the buildings. Reporter: Shadows and shade are a constant enemy. I've tripped a few times and have a mini heart attack when you trip going downstairs. Reporter: So are faces which makes it hard for her to recognize her friends. There's been times I didn't know it was her, so I might make a little comment to say something. You didn't know that, did you? No, I never noticed that. When she was smiling, I could tell, but I see where her eyes are, but I can't see her eyes. Reporter: Heather uses a variety of tools. High tech. I have to invert for writing. Reporter: And low tech. My felt tip marker. My recipe's on my iPad so I can see it better. Reporter: But her biggest, her fiance, Christian. I've tried to look at him super close in the sun. Never really seen him, never been able to see a blemish or anything like that on his face. She always looks for the best in things. I want to be with her knowing that she's like that. I'm scared to not be able to see like Christian's face or our future children's faces. I wanted to be a mother all my so I want to be able to at least see their faces. Reporter: She was giving up hope when suddenly, she got a call. How did you get the word? I was asleep one morning and my dad called me. You awake? There's a thing on the TV, a cure for the blind, check it out. I think it's your disease. Reporter: It all started in this medical lab in Philadelphia with a groundbreaking new drug. For almost as long as they've been married, doctors Jean Bennett and Albert Mcguire, an eye surgeon, have been searching for a cure for genetic blindness. At that point in time the technology did not exist. Reporter: So if you were older, more experienced in this you would have known it was almost impossible. It's a good thing not knowing, because it would have been daunting. This is ridiculous. Reporter: Eventually, technology caught up. Dr. Bennett figured out a way to replace the gene that cause people like Heather and Monroe to go blind. Rpe 65. And Dr. Mcguire figured out how to inject the replacement into the patient's eye. The treatment now called luxturna. Then the next thing, wouldn't it be great if we could make blind children see? Reporter: Heather quickly signed up. Dr. Aaron Miguel will handle both surgeries. It is changing the course of the disease in a way we never saw before. Reporter: While both operations will be the same, results will be unique, age is a big factor. Monroe who is 5 may have more improvement and stable improvements over the long term, compared to Heather because of age may have lost more tissue. Reporter: Making the decision to risk what eyesight they have left is a big deal. Enter Christian Gardino. when I had you Reporter: Millions fell in love with him on season 12 of America's got talent, where he earned the coveted golden buzzer. When he was 12, Christian was chosen for Dr. Bennett and Dr. Mcguire's clinical trial. Was this a tough call? No. Reporter: What's the very first thing that you saw? I think it was my parents. Seeing my parents' face in detail, that was pretty Reporter: Before surgery, doctors recorded Christian trying to navigate a maze. You can see he has trouble staying in line, missing steps and running into barriers. After treatment, Christian saw things he'd never seen before, maneuvering around the obstacles, flawlessly. Massive, massive improvement, yeah. I'd take anything of what I have now over what I had before. Reporter: After months of waiting, it's Heather's chance to gain something she's always My hopes that I'm afraid to admit. To see a star and to be able to see Christian's face, actually see his eye color. I have a appointment at the vision center. Reporter: It will take two operations. Her left eye is up first. Heather's dad and step mom arrive. It's still hard. Oh, boy. My vision doesn't define who I am, but it's kind of made me who I am. I don't know how to not be the blind girl. Reporter: The operation itself will last about an hour. The luxturna goes underneath the retina. We use a very fine needle to deliver a small amount of this medicine underneath the retina. The retsna is only a quarter of a millimeter thick. No room for error. Even a small tremor can make a difference. Reporter: Dr. Miguel threads that delicate needle and Heather's father and fiance wait. I love you, baby. Can I have a kiss good-bye? I'll see you when you wake up, okay? Reporter: For Monroe, the surgery could be a chance at a normal childhood. She was afraid. She didn't know what to expect, but I do know she was scared. Just to see my baby willed back, praying that I'm making the right decision. Praying that the end result is going to be what we hope it will be. Reporter: Post surgery, they both have to wear a protective eye patch for 24 hours, and then that first moment of truth. Once she took that patch off of her eye, what did she see? Nothing. Reporter: Black? Yeah, it was black. She kept saying I couldn't see, I can't see. We're going to check your I'm scared to open it. You see my hand moving here?
This transcript has been automatically generated and may not be 100% accurate.