LAS VEGAS – When Brantley Frazier was lying in a hospital bed, waiting for his heart to start working, he had “Uno” for good luck.

“Uno” was Brantley’s first sock monkey, and his dad, Justin Frazier, kept watch over it as he watched doctors try to make the transplanted organ fit correctly into his chest.

Just two weeks into his life, Brantley had gotten a new heart when they discovered holes in the original one. He was born with a congenital birth defect, hypoplastic left heart syndrome, that prevents the oxygen-rich blood from circulating the body.

Frazier could see the new heart beating through his son’s open chest, filled with all sorts of tubes to allow breathing.

At the time, Brantley was the second youngest heart transplant recipient in the world. He weighed around six pounds. The next 18 months, Frazier and his wife lived in the hospital as their son struggled to get healthy.

The heart transplant was just part of the fight. Frazier said the medication Brantley took for his heart gave him post-transplant lymphoproliferative disease (PTLD), a cancer that often develops in transplant recipients. He underwent chemotherapy as he recovered from the transplant.

Brantley is now six years old. He’s three feet tall and weighs 32 pounds. He’s struggled with seizures in the past, but is doing better than ever. He’s quite precocious, as well.

“Actually, he’ll correct me on that,” Frazier (10-2 MMA, 0-0 UFC) told reporters at a media day for his upcoming fight at The Ultimate Fighter 28 Finale. “He’s 32.3 pounds right now. He’s gained some weight, so he’s pumped about that.

“He’s by far the smartest six-year-old you’ll ever see. He will argue and carry on like you wouldn’t believe.”

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Frazier read letters from Brantley to motivate him while he was on “The Ultimate Fighter” reality show, which kept him secluded in a house with 15 other fighters for several weeks with no ability to communicate with his family or the outside world. He revealed another letter is waiting for him at the conclusion of weigh-ins for his fight with fellow finalist Juan Espino (8-1 MMA, 0-0 UFC).

Before he left for the final two weeks of training for his fight against Espino, which takes place Friday at Palms Theater in Las Vegas and airs on FS1 following early prelims on UFC Fight Pass, he got one with one last message of encouragement.

“Dad, I know you’re going to kick butt,” Brantley wrote. “You don’t have any other option to do that. But just for good luck, I’m going to send ‘Uno’ with you for good luck, because he was there for me when I needed good luck.”

On the cusp of the biggest fight of his career, Frazier doesn’t feel like he needs luck to beat Espino. But any word from his son reminds him to fight as hard as he possibly can. He has a good example waiting at home.

For more on The Ultimate Fighter 28 Finale, check out the UFC Rumors section of the site.