Some of the most watched and listened to music on YouTube and YouTube Music disappeared on Saturday, replaced by a short message:

Video unavailable

This video contains content from SESAC. It is not available in your country.

That is probably not what people expected when they tried to stream Adele’s “Rolling in the Deep” or other songs from artists as varied as Kendrick Lamar, Britney Spears, Green Day, Kanye West, and Burna Boy.

This leaves two questions: what is SESAC, and when is the music coming back?

SESAC stands for the Society of European Stage Authors and Composers, which has been around since 1930 and, according to its website, “…currently licenses the public performance of more than 1.5 million songs on behalf of its 15,000+ affiliated songwriters, composers, and music publishers.”

It’s smaller than similar organizations like BMI and ASCAP, but SESAC lists many big-name artists in its portfolio. In 2017, it was acquired by the private equity firm Blackstone.

Answering the question of when the SESAC music ban on YouTube and YouTube Music will end, or even what music has been removed, is a little harder.

SESAC has a searchable database of its repertory, although not all the songs listed there appear to have been removed. There’s even a 44,267-page PDF available on the site if you’d like to read the complete list. But not all songs were affected in the same way. While one listing for Kanye West’s “Power” is blocked, as of this writing the music video version continues to stream just fine.

The licensing organization hasn’t commented publicly, but YouTube did respond to an inquiry from The Verge, as spokesperson Mariana de Felice writes:

We have held good faith negotiations with SESAC to renew our existing deal. Unfortunately, despite our best efforts, we were unable to reach an equitable agreement before its expiration. We take copyright very seriously and as a result, content represented by SESAC is no longer available on YouTube in the US. We are in active conversations with SESAC and are hoping to reach a new deal as soon as possible.

According to an unnamed source cited by Variety, this could be a negotiating tactic by YouTube because, they say, “the previous deal actually does not expire until next week.”