United Airlines has become the first US airline to introduce a non-binary gender booking option.
Customers will now be able to identify themselves as 'Mx', alongside the existing options of female, male, unspecified and undisclosed when booking a flight.
Announcing the move on Twitter, the company wrote: "Fly how you identify. Our new non-binary gender options are now available."
The airline has also taken steps to train employees about "preferred pronouns" and the "persistence of gender norms".
United's chief customer officer, Toby Enqvist, said: "United is excited to share with our customers, whether they identify along the binary of male or female or not, that we are taking the steps to exhibit our care for them while also providing additional employee training to make us even more welcoming for all customers and employees."
The news has sparked a mixed reaction online, with one Twitter user writing: "Why include gender at all in the process? We're all just packing into a tiny sky tube together for a few hours."
Another wrote: "A customer is a customer, giving them the respect of addressing them how they prefer is just decent."