In real life, the Crowley children were diagnosed with the most severe form in infancy. In the film they were diagnosed later in childhood and might have been expected to have a somewhat milder, but still fatal, form.
Venture capitalist John Crowley (played by Brendan Fraser) has two children with the neuromuscular disorder Pompe disease. The genetic condition, which is described as uniformly fatal in its most severe form, creates a deficiency of an enzyme that converts stored sugar -- or glycogen -- to usable glucose. Without that enzyme, the glycogen builds up in the body and damages the heart and other muscles. Searching for a way to help his children, John finds a scientist, Dr. Robert Stonehill ( Harrison Ford), who is developing a treatment.