The National Music Publishers Association on Monday fired an opening salvo at lyric sites that it believes have not obtained licenses to publish those lyrics, including Rap Genius, a high-flying New York startup that last year landed a $15 million investment from Silicon Valley venture firm Andreessen Horowitz.

The NMPA said it has sent take-down notices to 50 sites identified in an October report by University of Georgia researcher David Lowery as likely not having licenses to publish lyrics. The notices demand that the sites obtain licenses or remove copyrighted lyrics from their sites.

Rap Genius Co-Founder Ilan Zechory said his New York company had not heard from the NMPA, "but we can't wait to have a conversation with them about how all writers can participate in and benefit from the Rap Genius knowledge project."

Zechory added, "Rap Genius is so much more than a lyrics site! The lyrics sites the NMPA refers to simply display song lyrics, while Rap Genius has crowdsourced annotations that give context to all the lyrics line by line, and tens of thousands of verified annotations directly from writers and performers. These layers of context and meaning transform a static, flat lyric page into an interactive, vibrant art experience created by a community of volunteer scholars. Furthermore, music is only a small part of what we do. Rap Genius is an interactive encyclopedia for annotation of all texts - anyone can upload and annotate texts relating to music, news, literature, religion, science, their personal lives, or anything else they want."

David Israelite, Chief Executive of the NMPA, said his organization was not targetting fan sites, but rather commercial websites.

"This is not a campaign against personal blogs, fan sites, or the many websites that provide lyrics legally," Israelite said. Rather, the NMPA "is targeting fifty sites that engage in blatant illegal behavior."

The group claims that more than five million searches for "lyrics" occur each day on Google, and that over 50% of all lyric page views are on unlicensed lyric sites., which displays advertising, claims on its site that it has 12 million unique visitors a month. The site did not immediately respond to a request for comment. 

Israelite said that the take-down notices are a precursor to filing copyright infringement lawsuits against sites that continue to publish song lyrics that they don't have the licenses to. His organization, on behalf of Warner Chappell Music, Peermusic and Bug Music, last year won a $6.6 million judgement against LiveUniverse, a company founded by Brad Greenspan that operated unlicensed lyrics sites. In 2010, the NMPA successfully sued Motive Force, a company that operated LyricWiki, and received an undisclosed amount of "funds associated with the exploitation of the unauthorized content."

It's unclear how much money is at stake. Rap Genius, for example, currently does not have advertising. Lowery's study, however, points to the potential for lyrics to generate large amounts of advertising dollars in a digital environment.

"Based on the popularity of lyric searches, it is possible that unlike the sound recording business, the lyric business may be more valuable in the Internet age," Lowery wrote in his Oct. 22 report. "Indeed, the vast majority of these websites seem to have well established monetization schemes based on advertising. Many of the sites appear to have accounts with major online advertising exchanges and prominently feature advertising from!major brands. There are even companies that appear to specialize in matching specific lyrics to key demographics for advertisers."

Here's a list of the 50 sites that have been sent take-down notices by the NMPA.