Secretary of State Marco Rubio, in a memo reviewed by NBC News, said additional guidance would be issued on existing passports containing an “X” sex marker.

The Guardian reports that the U.S. State Department has frozen all applications for passports with “X” sex markers and changes to gender identity on existing passports. The move follows an executive order signed this week by President Donald Trump declaring there are only two “immutable biological classifications” – male and female.

At the heart of the issue is the definition of sex and gender. Rubio's email states, "The policy of the United States is that an individual's sex is not changeable". This stance is reinforced by Trump's executive mandate.