A group of Republican senators is asking Amazon.com Inc. to explain why it recently removed a book regarding transgender issues that had been on sale on the platform for roughly three years.

In a letter to Amazon Chief Executive Jeff Bezos dated on Wednesday, Sens. Marco Rubio of Florida, Mike Lee of Utah, Mike Braun of Indiana and Josh Hawley of Missouri said a book by conservative scholar Ryan T. Anderson, “When Harry Became Sally: Responding to the Transgender Moment,” was no longer available on Amazon nor on its Kindle and Audible platforms.

An Amazon spokeswoman didn’t need to comment regarding the letter. The company declined to provide details regarding its decision, saying in a statement earlier Wednesday that it reserved the right not to sell specific content based on its content guidelines for books. 

In their letter, the senators wrote that Amazon’s move “openly signaled to conservative Americans that their views are not welcome on its platforms.” They requested the company provide documentation explaining its decision, including whether the book violated an Amazon policy.