The bond between sisters is sometimes hard to explain.
Mikayla and little sis Morgan loved to laugh and play together.
They were inseparable.
Then, one day, four-year-old Mikayla started having seizures. She ended up in two medicated comas; half of her brain was removed; and she passed away in six short months.
Mikayla had Alpers', a disease affecting the central nervous system, and her mitochondria. It is genetic and later testing showed Morgan had it, too.
"You always think your miracle is not going to come — especially after you lose your first child," said mom Dasa Gain. "So you get thrown into a middle ground where you have to fight for one and grieve for the other. That's a very hard thing to do."
So, Gain fought.
She found a new trial at Stanford University for a drug called EPI. The drug is like a battery, trying to recharge failing cells.
At the age of three, Morgan was the first child in the world with Alpers' to try this treatment.
"There are a lot of things that if Mikayla hadn't gone through them, we wouldn't have known how to take care of Morgan. So in a way, she saved her sister," Gain said.
It's now been three years since Morgan started EPI treatment, and she is virtually seizure-free. She can eat without a feeding tube. She plays, crawls, and communicates now, all improvements her mom credits to EPI.
"The day we found out Morgan was going to get on the EPI trial, there was a balloon in the side of the room. We didn't have the fan or anything on," Gain recalled. "It was a little balloon that said, 'I love you, I'm going to try not to cry.' That balloon floated across Morgan's bed when we went in there and found out she got on the trial."
So with big sister in mind, Morgan let go of balloons in California after her first treatment.
"I think she's right there on the verge of hope for everybody," Gain said.
Charlotte Davidson, 20 months old, is now following in Morgan's footsteps. This Elmo-loving, girly girl is taking part in EPI trials now spreading to multiple cities, including Houston, at Texas Children's Hospital.
Charlotte has Leigh's disease, a mitochondrial disorder in which the cells in the body don't produce the right energy.
Dr. Fernando Scaglia, a medical geneticist with Texas Children's Hospital, said this trial is showing great promise.
"We may be able to slow down the progression of the disease and maybe improve the outcome," he said.
For the first six months of the trial, it is truly a guessing game. Is it the placebo or the EPI? The families don't know, and neither do the doctors.
But this is a big day. Charlotte will get EPI now — its just a matter of what dose.
"This is the moment we've been waiting for six months, every single day," said dad Eric Davidson.
As with Morgan, the bond between sisters plays a part here, too. Ashley Davidson credits her sister for getting Charlotte on this trial.
"She paved the way for us," said Ashley.
In part because she's going through it, too.
Ashley has two nephews already taking part in the trial. Both are now showing outward improvements in movement and understanding.
"It helps to have someone understand how you feel when your child misses the milestones," Ashley Davidson said. "When they do the little things, like say, 'Mommy,' they understand what a big thing that is. You have someone celebrate with you as much as you celebrate."
Dr. Scaglia said the success of the trail won't truly be known until it's over. And he warns that mitochondrial diseases are unpredictable. But he calls EPI the biggest break in 10 years.
"This has the potential to help people in the U.S. and around the world, as we have seen many requests from many families from all continents," Dr. Scaglia said.
Not only Leigh's and Alpers patients, but victims of other diseases may one day benefit as well.
"Could it be Parkinson's? Could it be Lou Gehrig's disease, ALS? Some conditions where you see neurodegeneration, where you see regression," Dr. Scaglia said.
"This could be the answer, or this could be the bridge to the answer for a cure," Ashley Davidson said.
"You're never going to give up... never," Eric Davidson added.
And yet one selfless goal to help others.
"A lot of kids grow up," Dasa Gain said. "We see them out there going to graduation and soccer games... doing things their families are proud of. But for me, my kids are helping save other lives."