Sen. Thad Cochran (R-MS) to Retire. Chair of Senate Appropriations

Sen. Thad Cochran, the senior Republican Senator from Mississippi said on Monday that he will retire from the Senate on April 1 saying his "health has become an ongoing challenge." 
"I intend to fulfill my responsibilities and commitments to the people of Mississippi and the Senate through the completion of the 2018 appropriations cycle, after which I will formally retire from the U.S. Senate," he said in a statement. 
Cochran is the chairman of the Appropriations Committee.  Sen. Richard Shelby, (R-Alabama) is expected to take over the chairman role. 

A special election will be held in November to fill the retiring Senator's seat. Until that time, the governor of Mississippi will appoint a replacement Senator.