Brazilian UFC flyweight Allan Nascimento has died after suffering an apparent heart attack in his sleep, the organization announced. He was 34.

Nascimento was found unresponsive Monday morning, according to the UFC, and was pronounced dead at the scene “despite the efforts of the responding medical team.”

He is said to be survived by his wife and two children.

“Our thoughts and deepest condolences are with Allan’s family, friends, teammates, and loved ones during this incredibly difficult time,” UFC wrote on social media.

At 5 foot 9 and 126 pounds, Nascimento built a reputation “as a skilled competitor and consummate professional,” according to a tribute posted on the UFC site, while compiling a 4-2 record in the Octagon and 22-7 overall (16 wins by submission).

Nascimento started training at the famed Chute Boxe Diego Lima academy in his hometown of São Paulo at age 15. It was there that he was given his nickname, Puro Osso, which is Portuguese slang for “skin and bones.”

“When I joined my academy, I was extremely thin,” Nascimento told the website Sherdog in 2021. “I weighed nothing. As soon as people saw me, they called me ‘Puro Osso.’ The nickname stuck.”

Nascimento’s first MMA fight was in 2011, with his UFC debut coming 10 years later. His final bout was a split decision loss to Mitch Raposo at UFC Fight Night 279 in Las Vegas.

On Monday, former UFC lightweight champion Charles Oliveira posted several tributes to his close friend and Chute Boxe Diego Lima teammate on his Instagram story.

“Today I lost a brother that fighting gave me,” Oliveira wrote in Portuguese. “Thank you for always being there, for sharing the mat and the corner, and for your words. I’m just so grateful for having you by my side, brother, in training, in the corner and when hanging out. I love you, kid.”

Fellow Brazilian UFC fighter Caio Borralho commented on UFC’s Instagram post about Nascimento’s death: “He was one of the most kind fighters that’s I’ve ever met! Polite, good energy and always in good spirits! He will be missed.”

UFC Brazil reporter Evelyn Rodrigues wrote in the comments: “One of the kindest souls I’ve ever had the privilege of knowing through MMA. Rest in peace, Allan.”