Kate Brown is almost 2 years old. She seemed fine when she was born, sweet and quiet, good at nursing and sleeping.

"She was just a dream baby in the beginning," says her mother, Rachael. "Looking back now I see all the signs — hindsight is kind of cruel in that way. But we didn't know — we had no idea we were carriers for this disease. I had no idea the disease even existed."

The disease is called spinal muscular atrophy. It's a rare gene defect; the severe form affects about 300 American infants a year. And until recently, it was a death sentence for babies like Kate.

By the time she was 10 weeks old, she was going downhill fast. Her limbs were floppy, and she hadn't gained any weight in over a month. The spinal muscular atrophy was destroying motor neurons, the nerve cells that control movements like breathing and swallowing.

Rachael and her husband Patrick were preparing for Kate's baptism when they got a call from their pediatrician, Dr. Rosa Rincon.

"She said to me: I have been up all night. I have been reading. Something is not right with your daughter," Rachael remembers. "You need to go to Children's Hospital. I have called them. Go now."

If Kate had been diagnosed even just a few years earlier, nothing could have been done for her. But she was eligible for a new drug, called Spinraza, that compensates for the defective gene. It's not a cure, but it can make a big difference.

Hanging on a wall in the dining room of the Brown family home in West Boylston, Massachusetts, is a framed label from the Spinraza box that held Kate's first dose.

"To other people it's a piece of cardboard, but to me, it's hope, and it's my child's life," Rachael says. "Because without this, I know Kate would not be alive today. We have options, when so many people before us didn't. So many people, so many families, took risks to find this drug. And without them, I wouldn't have my baby."

Kate Brown, who has the rare genetic disease spinal muscular atrophy, or SMA, has on her advocacy hat. (Courtesy Brown family)

For all its benefits, Spinraza caused some sticker shock when it came out in late 2016. Its cost: $750,000 for the first dose, and about half that every year for the patient's lifetime. Last year, a gene therapy treatment called Luxturna, for a genetic form of blindness, came out with a price tag of $850,000.

Now, a new treatment called Zolgensma (pronounced zohl-JEN-smah), which replaces the gene that's defective in spinal muscular atrophy, is expected to break that price record this month. The FDA is expected to approve Zolgensma sometime in May, and the biopharma company Novartis is then expected to announce the price.

Novartis executives have made clear it will be in the seven figures. Dave Lennon, president of AveXis, the company that developed Zolgensma and was bought last year by Novartis, says the one-time treatment would be cost-effective even at $4 or $5 million, Reuters reported.

"We're betting $2 million, within a small amount of that range," predicts Michael Sherman, chief medical officer of the health insurer Harvard Pilgrim Health Care.

In the great national debate under way about drug prices, Zolgensma is sure to be a hot topic. But not a simple one: It illustrates the complex balance between the good news — saving children from a horrible disease — and the bad news — stratospheric prices.

"This has been anticipated to be the most expensive treatment ever launched in the United States and perhaps in the world — for several reasons," says Dr. Steve Pearson. "One is because it is creating tremendously improved outcomes. It's taking an always-fatal condition, and it could be a true cure."