Prince Frederik of Luxembourg, the youngest son of Prince Robert and Princess Julie, died from a rare genetic condition called POLG Mitochondrial disease. He was 22.

On the website for the POLG Foundation, Prince Robert announced that Frederik died on March 1. Frederik established the foundation to support research and find a cure for the illness.

“It is with a very heavy heart that my wife and I would like to inform you of the passing of our son,” Prince Robert said in a statement.

Prince Robert referred to his son as the family’s “Superhero,” and he highlighted the many achievements of the POLG Foundation, which Frederik helped found.

The United Mitochondrial Disease Foundation states that mutations in the POLG gene are the cause of mitochondrial ailment POLG disease. It mostly affects the liver, muscles, nerves, brain, and other organs and can also impair vision.

Prince Robert said that his son was “my Superhero… and to so very many good friends.” He “was born with a special capacity for positivity, joy, and determination.”

The father said that his late son had unmatched “social skills,” “emotional intelligence and compassion,” and an “unbounded sense of justice, fairness and decency.”

The 56-year-old prince added that Prince Frederik “called us into his room to speak to him for one last time” on February 28, Rare Disease Day.

Prince Robert said that his 22-year-old son “found the strength and the courage to say goodbye” to his “his brother, Alexander; his sister, Charlotte”, and him.

Prince Frederik also bid farewell to his three cousins, “Charly, Louis, and Donall; his brother-in-law, Mansour”, and his aunt “Charlotte and Uncle Mark.”

He had “already spoken all that was in his heart to his extraordinary mother,” Prince Robert continued, talking about his 58-year-old wife Julie.

Prince Robert added that Frederik’s goodbye remarks were “kind, wise, and instructive,” adding that his son had “a final long-standing family joke.”

Although Prince Frederik had POLG mitochondrial disease from birth, his father Robert claimed that his symptoms did not become apparent until he was 14 years old.

Treatments for POLG conditions can help control symptoms and enhance quality of life, but there is currently no cure for the ailment.