What do you get when you mix Jay-Z with Linkin Park?

Mash-ups (search) -- remixes that cleverly craft two songs together -- have been a growing underground phenomenon for years, and are big in the U.K. and the rest of Europe, but now, with the help of MTV, these gems of the DJ world are moving into the national spotlight.

Last summer, as part of its "Ultimate Mash-Ups" (search) project, MTV put Linkin Park and Jay-Z onstage in Las Vegas, where the two stars did live mash-ups of their music, blending Jay-Z's "Izzo" with Linkin Park's "In the End," for example.

And the resulting album, "MTV Ultimate Mash-Ups Presents Jay-Z/Linkin Park: Collision Course," (search) debuted at No. 1 last month.

Boosted by the record's success, the network announced that it would produce five more "Ultimate Mash-Ups" specials, and is talking to artists such as Green Day, Ludacris, Kanye West, Coldplay, 50 Cent, the Beastie Boys, Snoop Dogg and Missy Elliot.

Imagine Chris Martin of Coldplay putting up his Brit-pop up against the gangsta rap of 50 Cent. Or Green Day's Billie Joe Armstrong doing it doggy-style with Snoop Dogg.

In fact, it's becoming the latest bar talk — guessing which artist will team up with whom.

Mash-ups are a natural result of the diverse tastes of today's music fans. Kids don't just stick to one musical genre anymore — they'll click onto a Kanye West hip-hop tune in their iPod one minute and then flip to a Strokes rocker the next.

With that in mind, DJs take tunes from unlikely suspects to come up with surprising results.

Earlier Internet hits include "Oops! Slim Shady Did It Again," which mixed Eminem's "The Real Slim Shady" and Britney Spears' "Oops! . . . I Did It Again"; and "Smells Like Booty," which crashed Nirvana's "Smells Like Teen Spirit" and Destiny's Child's "Bootylicious."

Last year, producer Danger Mouse helped push the trend upward, creating an Internet phenomenon with "The Grey Album," which put The Beatles' "White Album" up against Jay-Z's "The Black Album." Jay-Z had no complaints, but EMI, which holds The Beatles' copyrights, put out a cease-and-desist order.

It didn't matter; Pandora's box had been opened and kids began trading homemade mash-ups on the Web. MTV execs noticed mash-ups being passed around on the MTV.com message boards.

"We took it from the indie underground and are trying a mainstream approach," says Tom Calderone, MTV executive vice president of music and talent programming.

Even David Bowie was inspired by "The Grey Album." The Thin White Duke held a mash-up contest on his Web site when he released his latest album last year.

Rights issues are the reason MTV had to go live with its mash-up concept, instead of doing video mash-ups. "It's illegal. We just can't be illegal," Calderone says.

But one New York-based musician, Jeremy Brown, also known as DJ Reset, miraculously got the rights to all the players for "Frontin' on Debra," a sweet mix featuring Beck, Pharrell Williams and Jay-Z, to which he added his human beat box and live drums.

Beck heard the track on the radio (K-Rock in L.A. and New York were spinning it) and instead of quashing it, he helped Brown get clearance from the multiplatinum players. Interscope, Beck's label, then released the track on iTunes. It was the first mash-up to be sold on the site — and it became one of iTunes' hottest tracks.

Nick Barat, a New York-based DJ known as Catchdubs and an editor at Fader magazine, says mash-ups mixing different musical genres may introduce new styles to more people.

"It gives people a safe way to look at stuff they never would have looked at before," he says. He points out that the "The Grey Album" made it easy for rock fans to check out Jay-Z, and perhaps get turned on to hip-hop for the first time.

Brown says he enjoys bringing together different artists and introducing them to wider audiences. "That only adds to better harmony among different genres of music and people," he says.

But getting clearance is the exception, so mash-ups will continue to be passed around on the 'Net — at an even greater rate as kids everywhere see their favorite acts mashing on MTV.

"It's going to help take the art to the next level," says Brown.

Some kids are saving their allowances to buy turntables, instead of electric guitars, or uploading the necessary software on their computers to make their own monster mixes instead of learning Rolling Stones songs.

"Kids used to be bedroom rockers — now they're getting DJ starter kits," says Barat.

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