What happened to Julian Assange's Twitter account?

Julian Assange's Twitter account is gone.

"Sorry, that page doesn't exist!" is what Twitter says when a user deletes their own account, I've been told. However, that doesn't mean it's the case this time, especially when it's the account of somebody who is effectively a political prisoner.

@Wikileaks and @EmbassyCat are still up, but have no info on what's happened...