In the world of hip-hop few music executives have more influence than DJ Drama. His “Gangsta Grillz” compilations have helped define this decade’s Southern rap explosion. He has been instrumental in the careers of rappers like Young Jeezy and Lil Wayne. He appears on the cover of the March issue of the hip-hop magazine XXL, alongside his friend and business partner T.I., the top-selling rapper of 2006. And later this year DJ Drama is scheduled to make his Atlantic Records debut with “Gangsta Grillz: The Album.” Now DJ Drama is yet another symbol of the music industry’s turmoil and confusion. On Tuesday night he was arrested with Don Cannon, a protÃ©gÃ©. The police, working with the Recording Industry Association of America, raided his office, at 147 Walker Street in Atlanta. The association makes no distinction between counterfeit CDs and unlicensed compilations like those that DJ Drama is known for. So the police confiscated 81,000 discs, four vehicles, recording gear, and “other assets that are proceeds of a pattern of illegal activity,” said Chief Jeffrey C. Baker, from the Morrow, Ga., police department, which participated in the raid. DJ Drama (whose real name is Tyree Simmons) and Mr. Cannon were each charged with a felony violation of Georgia’s Racketeering Influenced Corrupt Organization law(known as RICO) and held on $100,000 bond. The compilations produced by DJ Drama and his protÃ©gÃ©s are known as mixtapes, though they appear on CDs, not cassettes. Mixtapes have become a vital part of the hip-hop world. They are often the only way for listeners to keep up with a genre that moves too quickly to be captured on albums. On a mixtape you can hear unreleased remixes, sneak previews from coming CDs, casual freestyle rhymes, never-to-be-released goofs. Mixtapes are, by definition, unregulated: DJs don’t get permission from record companies, and record companies have traditionally ignored and sometimes bankrolled mixtapes, reasoning that they serve as valuable promotional tools. And rappers have grown increasingly canny at using mixtapes to promote themselves. The career of 50 Cent has a lot to do with his mastery of the mixtape form, and now no serious rapper can afford to be absent from this market for too long. As mixtapes evolved from a street-corner phenomenon to a cornerstone of the hip-hop industry, record companies tried to figure out ways to cash in. Mixtape D.J.’s like DJ Clue, DJ Kay Slay and others have released major-label compilations full of tracks that abide by copyright rules. But it’s not easy to turn a mixtape into something you can legally sell: part of the fun is hearing rappers remake one another’s songs and respond to one another’s taunts; a great mixtape captures the controlled chaos that hip-hop thrives on. DJ Drama’s mixtapes are often great. He has turned “Gangsta Grillz” into a prestige brand: each is a carefully compiled disc, full of exclusive tracks, devoted to a single rapper who is also the host. Rappers often seem proud to be considered good enough for a “Gangsta Grillz” mixtape. On “Dedication,” the first of his two excellent “Gangsta Grillz” mixtapes, Lil Wayne announces, “I hooked up with dude, now we ’bout to make history.” The compilation showed off Lil Wayne more effectively than his albums ever had, and “Dedication” helped revive his career. When some unreleased tracks by T.I. leaked to the Internet, T.I. teamed up with DJ Drama for a pre-emptive strike: together, they created a mixtape called “The Leak.” As mixtapes have grown more popular, they have also grown easier to purchase, despite that official-sounding declaration – “For Promotional Use Only” – printed on every one. Sites like mixunit.com specialize in selling them, and big record shops and online stores have followed suit. As of yesterday DJ Drama was sitting in jail, but dozens of his unlicensed compilations were still available at the iTunes shop. Brad A. Buckles, executive vice president for anti-piracy at the Recording Industry Association of America, said, “A sound recording is either copyrighted or it’s not.” And he said the DJ Drama case, like most piracy cases, began with illegal product, which was then traced back to the distributor. Chief Baker said that before the raid, DJ Drama and Mr. Cannon were sent cease-and-desist letters from a local lawyer. There have been mixtape busts before: in 2005, five employees of Mondo Kim’s, in the East Village in New York, were jailed after the store was found to be selling unlicensed mixtapes. But the arrest of a figure as prominent as DJ Drama is unprecedented. Record companies usually portray the fight against piracy as a fight for artists’ rights, but this case complicates that argument: most of DJ Drama’s mixtapes begin with enthusiastic endorsements from the artists themselves. It also seems clear that mixtapes can actually bolster an artist’s sales. The most recent Lil Wayne solo album, “Tha Carter II” (Cash Money/Universal), sold more than a million copies, though none of its singles climbed any higher than No. 32 on Billboard’s Hot 100 chart. That’s an impressive feat, and it’s hard to imagine how he would have done it without help from a friendly pirate.