Fallout from last week's terror attacks has upended Broadway's most fundamental rule: Some shows will not go on.

With overall box office down nearly 80 percent last week, at least four Broadway shows will close this weekend, with others likely to follow.

The final curtain will come down Sunday on the revival of The Rocky Horror Show, the Tom Selleck-starrer A Thousand Clowns, the Irish import Stones in His Pockets and the comedy If You Ever Leave Me, I'm Going with You. (A fifth production, Blast!, announced two weeks ago that it would close on Sunday.)

In fact, some producers say they'd shutter the productions earlier if there wasn't an Actors' Equity rule requiring shows to give a week's notice.

The list is almost certain to grow longer as virtually every show has been impacted by the attacks.

Broadway went dark last Tuesday and Wednesday, meaning productions lost revenue from two evening performances and one matinee. When the curtains came up on Thursday, the houses were virtually empty.

Crowd-pleasing big-ticket revivals The Music Man, Kiss Me Kate and Chicago face a tough struggle to stay open. Tourist-destination productions like Les Miserables and The Phantom of the Opera didn't have many tourists to play to. And for the first time in its four-year engagement, Disney's The Lion King failed to sell out, filling to 70-percent capacity last week.

Even The Producers, the city's toughest ticket, suffered a bit. There were at least 150 cancellations per show last week, Daily Variety says. Although those tickets were snapped up by people willing to wait in line, there was also a substantial number of ticket holders who failed to show. For the week, the Mel Brooks musical did about half its usual $1 million business.

Meanwhile, the new Steven Sondheim musical, Assassins, which was set to begin previews and open in November, was indefinitely postponed.

Overall, the closures cost Broadway around $3 million, according to Variety. Things are so grim, that, during a press conference Tuesday, Mayor Rudy Giuliani said the city was scraping together funds to keep productions going.

Despite the troubled outlook, producers and performers have announced a benefit next month, with proceeds going to a victim-relief fund.