To look at photos of Christopher and Christian Johnson, you'd never know they have sickle cell disease.
But it's a reality their mother Tracey has dealt with every day since shortly the twins were born.
"In the beginning, it's very scary. you're afraid," Tracey told us.
Sickle cell occurs in about 1 of every 500 african-americans, 1 in 1-thousand Hispanic Americans, and in some other ethnic groups.
Red blood cells, which should be disc-shaped, become crescent or sickle-shaped.
In babies, the spleen desperately tries to filter them out.
"The spleen gets clogged up completely and doesn't work anymore," says Dr. Kwaku Ohene Frempong, the director emeritus of the sickle cell center at Children's Hospital of Philadelphia.
So it can't fight infections as it should, and sickle cell babies can get sick very, very fast from infections which would be routine for other babies.
"They just grow very very fast, and can kill the children," says Dr. Frempong.
Tracey says regular antibiotics, the medication hydroxyurea, and monthly blood filtration helped her boys have a fairly normal life.
These days, Christopher is even away, studying at a military boarding school.
Tracey says she doesn't coddle her twins.
"I have the same expectations that I have for my other child," she told us.
But doctors at Children's are now focusing on prevention, by identifying sickle cell babies in the womb.
The baby would receive healthy bone marrow cells from the mother or father.
Dr. Frempong says, "If this works, the healthy cells from the mother or the father will become the dominant cells in the baby. It will make the baby grow up having regular blood."
He says animal tests have been successful thus far, and human trials are about 2 years off.
If all goes well, Tracey says her grandchildren might never have to worry about sickle cell disease.