Passengers who choose to will soon be able to pick a gender option other than male or female when booking their plane tickets.

On Friday, United Airlines claimed it had become the first U.S. airline to update its booking systems to provide travelers with the choice to identify themselves as U, for undisclosed, or X, for unspecified when using the drop-down menus on its online forms. United has also begun to let customers select the title “Mx.” during booking and in their customer profile for the MileagePlus loyalty program.

The carrier is telling employees that “Mx.” is pronounced “mɪks / mix” and saying that a customer doesn’t need to produce identification to use that when interacting directly with the airline, according to an internal United communications memo to its flight attendants.

United’s memo also advises its flight crew that, “If you use an incorrect pronoun, the best thing to do is to say, ‘I apologize. What pronoun would you like me to use?'”

Delta Air Lines has said it is in the process of updating its booking interfaces in a similar move.

Blazing a Trail

Last month, Airlines for America, an industry trade group, said it had worked with the International Air Transport Association (IATA) to agree on standardized procedures. Airlines for America’s members, such as American Airlines, Alaska Airlines, JetBlue, and Southwest are updating their reservation processes, too.

The carriers are playing catch up, as several local, state, and national governments issue identification documents with non-binary gender choices. The U.S. Transportation Safety Administration already accepts non-binary gender markers on passports or other valid government-issued identification, and it wants the marker to match with what airline documents say.

British Airways and Air New Zealand have announced similar plans. Lufthansa Group said it is considering the issue.

United said it was working with advocacy group Trevor Project, Human Rights Campaign, and other organizations to train its employees on respectful ways to interact with coworkers and customers who don’t identify as male or female.

Photo Credit: A United Airlines customer service agent interacts with a passenger at George Bush Intercontinental Airport in Houston, Texas. United passengers who choose to can now pick a gender option other than male or female when booking tickets. United Airlines