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Cleavage is back, and it's not where Marilyn Monroe had it.

It's a step to the rear, behind the scenes, where the sun doesn't (usually) shine.

Yes, it's "plumber's butt." And it's the hottest new trend for the fashion, er, forward.

"Butt cleavage is definitely in," said Alex Douglass, associate fashion editor at Cosmopolitan magazine. "To have a little of your body falling out is extremely sexy."

Celebrities like Pamela Anderson, Rosanna Arquette and Sarah Michelle Geller are all fans of the "dangerously low" Frankie B. jeans. Then there's Jennifer Lopez, whose much-publicized backside has been partially credited with her rapid rise to fame.

"It's all over Hollywood," said Frankie B. representative Mya Walters.

And now it's out on the road. One of Britney Spears' backup singers, Annette Stamatelatos, has been "butting" out — on and off-stage — throughout their current tour.

"We're all flying out of our pants — Britney, me, the dancers," Stamatelatos said. But she added that visible crack is a bane, not a boon, of the low-rise jeans trend.

"It's sexy at the pre-point before the pop-out," Stamatelatos said. "Beyond that you get a little self-conscious."

Douglass agreed the cheeky fad is somewhat of an accident. "It's basically a trend by default. In some cases, women don't even know they're doing it."

Other fashionistas agreed that butt cleavage is a situation that is endured rather than styled.

"I was bending down to get my subway fare, wearing my low-riders, when I heard, 'Hey, I can see the crack of your a--,'" said one New York twenty-something who asked to remain anonymous. "I was so embarrassed."

Not everyone thinks the trend is sexy. Stacy Mayesh, senior market editor at Marie Claire magazine, said showing too much "crack" is "disgusting." "Showing the top before the crack is sexy. Showing the crack itself is a major faux pas," she said.

But if it's such a pain in the you-know-what, what makes it worth it? Well, low-riders show off a lot of midriff at a time when many women are slaving away at their abdominal muscles. The look also helps accentuate a tight and toned derriere.

Many women, in fact, are mooning the streets with pride. Intermix, a boutique that sells the revealing jeans, said the rear window is a sought-after look. And several underwear retailers — including the Gap — are now selling low-rise thongs to accommodate the trend.

"Some of the girls are learning how to walk and sit better to hide it," Intermix sales associate Maria Suarez said. "But most don't mind."

So, what do guys think? While some prefer their cleavage more up front, others seem open to new possibilities.

"Breast cleavage has a pretty good tenure — the history of the world, perhaps," said Jason Adams, contributing senior editor at Blender magazine. "But I don't think it's a choice. Perhaps we'd like to see a bit of both."