Spinal muscular atrophy: The clinical trial that is helping Sydney boy Xavier Harrison beat the odds

When Emily and Mitch Harrison celebrated their son Xavier's fifth birthday recently, it was a milestone they didn't take for granted.

The  Sydney boy was diagnosed with spinal muscular atrophy, known as the most common lethal genetic disease among infants, after a heel prick test at birth.

His parents feared their son would not make it past the age of two.

"It's the number one genetic killer at that time, at most Xavier would live to two years old, if that," Mrs Harrison told 9News.

"They were predicting that wouldn't be the case for Xavier."

Professor Michelle Farrar offered a clinical trial of oral drops called risdiplam.

The medicine helps the body produce more spinal motor neuron proteins by targeting the RNA from a gene that is effectively a back up to the mutated or miss