Luxembourg's Prince Frederik has died at the age of 22 from a rare genetic condition, his family has announced.
Frederick, the son of Prince Robert of Luxembourg and Princess Julie of Nassau, died on March 1 in Paris from PolG mitochondrial disease, his father said.
The late prince was a member of the world's only remaining sovereign grand duchy, meaning Luxembourg is the only independent nation ruled by a grand duke rather than a king or queen.
Prince Robert penned a heartfelt statement on the website of the POLG Foundation, an organisation Frederik founded to accelerate research into treatments for the rare genetic condition.
He wrote that his son "called us in to his room to speak to him for one last time" on February 28.
"In true Frederik fashion, he left us collectively with a final long-standing family joke," Prince Robert wrote.
"Even in his last moments, his humour, and his boundless compassion, compelled him to leave us with one last laugh... to cheer us all up."
Prince Robert described Frederik as the family's "superhero" and made note of the POLG Foundation's many accomplishments.
"Part of his superpower was his ability to inspire and to lead by example," he wrote.
"He has social skills like no other, an amazing sense of humour, an emotional intelligence and compassion that were off the charts, a sense of justice, fairness and decency that knows no bounds."
Frederik was born with PolG mitochondrial disease but was not diagnosed until he was 14, when his symptoms became more pronounced, his father said.
What is PolG disease?
According to the United Mitochondrial Disease Foundation, PolG disease is a mitochondrial disorder caused by mutations in the POLG gene. It primarily affects multiple organs, including the brain, nerves, muscles and liver, and can affect vision.
There is currently no cure for PolG disease, though treatments can help manage symptoms and improve quality of life.
Last year, Frederik spoke candidly about his illness in the The PolG Foundation film.
"When you're a kid, you have all these dreams, all these aspirations, these things you want to do, the possibilities are endless," he said in the film.
"I'm not sure I understood all the consequences of PolG when I first got diagnosed … it's more subtle, where slowly, the world is getting smaller and smaller."
"This is the battle that Frederik fought, and this is the burden that he had to carry throughout his life. He always did so with grace and with humour," his father wrote.
