On 26 August, Conor McGregor will jump out of the UFC octagon and into a boxing ring to take on Floyd Mayweather in a multi-million-dollar superfight in Las Vegas, Nevada.
It is the biggest example of a combat sportsman crossing codes and looking to compete at the very highest level, but it is not the first.
Boxers and mixed martial arts (MMA) fighters have faced off in the past, with mixed results. Here, BBC Sport looks back at three of the highest-profile examples.
Royce Gracie v Art Jimmerson – 12 November 1993
Back at UFC 1, American light-heavyweight boxer Art Jimmerson entered the very first UFC tournament riding a 15-fight win streak in his usual sport.
A national Golden Gloves middleweight champion as an amateur, Jimmerson took the bizarre decision of entering the octagon wearing a boxing glove on his left hand, with no glove on the right.
Against Brazilian jiu jitsu star Royce Gracie, he was in over his head. He had no answer to the Gracie’s grappling skills. Indeed, Jimmerson was so befuddled and worried after being taken to the mat that he tapped out before the Brazilian could apply a submission hold.
The legend goes that Jimmerson had chatted with future UFC ref John McCarthy before the fight and, after realising he was out of his depth, opted to wear just the one glove to ensure the referee would clearly see him tap out if he got into difficulty.
Jimmerson was subsequently saddled with the nickname ‘One Glove’, but took his loss in good humour and now works as head boxing coach at the UFC Gym in Torrance, California.
“The stand-up fighters had no idea what to do on the ground,” said Gracie years later. “It was almost unfair.”
Ray Mercer v Tim Sylvia – 13 June 2009
When Ray Mercer and Tim Sylvia agreed to fight in 2009, it was supposed to be a boxing match between two fading veterans of the combat sports world. Mercer was a former world boxing heavyweight champion, while Sylvia had held the UFC heavyweight title.
But after the fight was vetoed by New Jersey’s state athletic commission, the bout was rescheduled as an MMA contest, with a gentlemen’s agreement between the pair that they would effectively box each other, with no takedowns.
Despite being held in a cage and branded a mixed martial arts contest, Mercer made clear before the bout explained that he had no intention of doing anything other than throwing his hands.
“I want him to know straight up where I’m coming from,” he said. “I am a boxer, I’m not a MMA fighter.”
The fight didn’t last long.
Mercer waited through the opening nine seconds before unleashing a sledgehammer overhand right that sent the 6ft 8in Sylvia crashing to the canvas – and won the contest.
Randy Couture v James Toney – 28 August 2010
Former multi-weight world boxing champion James ‘Lights Out’ Toney is the biggest name from the sport of boxing to take on an MMA fighter, but once again it took place in a cage, rather than the squared circle.
Toney chased UFC president Dana White around the USA, insisting that his fighters couldn’t punch and demanding a contest with oen of them.
White dismissed his challenge, before he finally relented and booked him to face Randy ‘The Natural’ Couture.
The 47-year-old former UFC heavyweight and light-heavyweight champion was a former amateur wrestler who became the first man to hold UFC world titles in two weight classes.
White felt Toney had no chance of victory, saying: “Here’s what I expect; the bell rings, they come to the centre of the octagon, circle each other for a little bit, Randy double legs him, puts him against the fence, squashes his head and the fight is over.”
Toney responded by dismissing Couture’s threat, as he said: “If he tries to take me down, it’s going to be a short night for him. A very short night.”
As it turned out, both men were right.
Couture secured a takedown just 15 seconds into the contest, softened up Toney on the mat, then locked up an arm-triangle choke to force the boxer to tap out a submission after three minutes and 19 seconds. The Boston crowd chanted: “UFC! UFC!”
After the fight, White told reporters: “James Toney picked this fight, and he got a fight. Toney lasted longer than I thought he would.
“It’s unfair to put a guy in there with one discipline, even if he’s trained for several months.”
Afterwards, Couture explained dryly: “I wanted to give James Toney the total mixed martial arts experience.”
Boxers have made the full-time move transition to MMA. Current UFC fighter John Phillips boxed for Wales as an amateur and was part of the Commonwealth Games team before taking up MMA in 2005.
Holly Holm won multiple world titles in boxing before switching to MMA and causing a shock by knocking out Ronda Rousey with a head kick to win the UFC bantamweight title in 2015.
But the key thing to note is the direction of travel. The boxers have moved into MMA, not the other way around. That’s why the upcoming Mayweather v McGregor bout has additional intrigue, because the Irishman is going against the grain.
Never before has one of MMA’s best exponents, in their prime, stepped out of their comfort zone against arguably boxing’s pound-for-pound best for a contest under the Queensbury rules.
With the drawing power of both men and the sheer weight of public intrigue, August’s match in Vegas promises to produce a unique sporting spectacle.