Lyric sites like Rap Genius, the user-generated project that allows fans to explain and understand the meanings of rap songs, have been accused by the National Music Publishers Association of copyright infringement.

The Chief Executive of the NMPA, David Israelite, said that around 50 sites engaging in “blatant illegal behavior” had been sent take-down notices. If these sites- including Rap Genius, Lyrics Mania and Poem Hunter- do not take action to acquire licenses to publish song lyrics, Israelite says that copyright infringement lawsuits will follow.

The NMPA’s move follows a report, by David Lowery of the University of Georgia, which identified the sites as hosting lyrical content to which they apparently did not own the rights. In his report, he claimed that “the vast majority of these websites seem to have well established monetization schemes based on advertising,” thus allegedly profiting from illegally published material.

“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,” he said.

Rap Genius, created in 2009 by three Yale students, is often popular with artists who want to better explain their own lyrics. Nas, Kendrick Lamar and RZA are amongst the many verified users on the site.