CLEVELAND — LeBron James put his mom and then the Boston Celtics in their places.

James scored 21 points, delivering a devastating dunk over a defenseless Kevin Garnett in the final two minutes, as the Cleveland Cavaliers beat the homesick Celtics, 88-77, in Game 4 last night to tie the best-of-seven series at 2-2.

Still stuck in a shooting slump, James dominated down the stretch and finished with 13 assists — four in the fourth quarter. The Cavaliers, too, showed that they can play stingy defense, holding the Celtics — the NBA's best defensive team — to just 12 points in the final period.

"That's what we needed to do," said James, whose mother, Gloria, came to her son's defense in the first quarter after he got tangled with Paul Pierce and Garnett.

Boston dropped to 0-5 on the road in the postseason, a stunning slip for a team that went 31-10 outside its home floor during the regular season. During a short stay in Ohio, the Celtics lost their momentum in the series but will now head home, where they went 35-6 before the playoffs started.

Afterward, Celtics players were at a loss to explain their travel difficulties.

"It's hard to say. I have no answer for it," forward Ray Allen said.

Game 5 is tomorrow night in Boston, and Game 6 will be back in Cleveland on Friday.

James was just 7 for 20 from the floor, but he did everything else for the Cavs, who are attempting to overcome an 0-2 deficit for the second time in two years.

In the final 8:45, James had four assists, a 3-pointer and a right-handed dunk that rattled Quicken Loans Arena and became the signature moment of this series.

With the Cavs leading 82-75, James drove past Pierce on a screen near the foul line, head faked past James Posey and then posterized Garnett, the league's defensive player of the year. As Cavaliers fans erupted, a scowling James stormed back on defense.

"I just wanted to be aggressive," James said. "I hadn't had a play like that all series. There was a lot built up."

After a timeout, the dunk was shown on the videoboard at least six times, giving Cleveland fans more chances to ooh and ahh at a play they won't soon forget. The Celtics may have a tough time erasing it from their minds.

"He can dunk," Celtics coach Doc Rivers said. "Especially if you give him a running start at the basket. It's probably going to be a pretty good dunk and he's so darn powerful that once he gets up there, there's not a lot you can do.

"I hate the fact that he got to the basket."

The Cavaliers were awed by James' stuff.

"That was incredible," Joe Smith said. "I've seen him do some crazy things since I've been here."

Although their star is shooting only 20 of 78 (26 percent) from the floor, the only numbers the Cavs care about are the pair of 2s that show this series is as tight as possible.

Garnett scored 15 points, but only two in the second half, as Cavaliers forward Anderson Varejao harassed Boston's All-Star all night. Allen had 15 points and Pierce 13, but the Celtics' Big Three were only a combined 16 of 40.

"We have to play better under stress," Rivers said. "I call them 'hero' shots and I thought we took a lot of those instead of just stressing what we do."

James missed his first six shots in the fourth before draining a 3-pointer in front of Boston's bench with 3:17 remaining to give the Cavs a 79-73 lead. After the ball swished through the net, James made an it's-about-time shrug.

After a bucket by Pierce, James dished to Daniel Gibson for a backbreaking 3-pointer as the Cavs opened a seven-point lead.

"They shot two big threes that were daggers," Allen said.

At the other end of the floor, the Cavs swarmed all over the Celtics, giving the league's best defensive team a dose of what they usually do to opponents. Cleveland contested every shot, and even when the Celtics had open looks, they missed.