Deputy Attorney General Rod Rosenstein has verbally resigned to Chief of Staff John Kelly in anticipation of being fired by President Trump, according to a source with direct knowledge. Per a second source with direct knowledge: “He’s expecting to be fired,” so he plans to step down.

Background: Rosenstein talked last year invoking the 25th Amendment and wearing a wire during Trump meetings, the N.Y. Times' Adam Goldman and Michael S. Schmidt reported last week. He denied both allegations.
https://www.axios.com/rod-rosenstein...c9e32ff43.html

The Mueller investigation could be about to come to a head... depending on who his replacement is.