United Airlines planes are pictured at George Bush Intercontinental Airport in Houston on March 18. (Loren Elliott/Reuters)

United Airlines travelers will be able to choose a gender option other than male or female when booking tickets, the airline announced Friday.

The move makes United the first U.S. airline to offer non-binary gender options for booking, following efforts in a growing number of states to begin issuing gender-neutral identification cards.

Customers will be able to identify themselves as male, female, undisclosed or an unspecified “X,” corresponding with the gender listed on their passports or identification cards, the airline said in a news release. Travelers will also be able to select the gender-neutral title “Mx.” during booking and in MileagePlus customer profiles.

“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,” United’s Chief Customer Officer Toby Enqvist said in a statement.

Last month, several other major airlines, including American, Delta, Southwest, Alaska and JetBlue, also indicated that they are in the process of adding non-binary gender options to their booking channels, the Associated Press reported.

United’s announcement Friday was met with praise from advocates for the transgender and non-binary community. Gillian Branstetter, a spokeswoman for the National Center for Transgender Equality, applauded the airline for adding gender options “that are reflective of the diversity of their passengers.”

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“Non-binary people face unnecessary, invasive, and discriminatory scrutiny by airlines, airports, and security services alike,” Branstetter said in a statement. “These changes by United and other airlines is in line with the growing number of states who offer gender neutral designations on IDs and is an important step toward ensuring safe and smooth travel for all passengers regardless of their gender.”

In rolling out these changes, United said it has also launched employee training initiatives with the help of the Human Rights Campaign and the Trevor Project, an organization that focuses on crisis intervention for LGBTQ youth. The initiatives include teaching employees about preferred pronouns and other aspects of LGBT competency in the workplace, United said in a statement.

A growing number of states and jurisdictions — including California, Minnesota, Oregon and the District — have begun offering gender-neutral options on identification cards. In Maryland earlier this month, lawmakers advanced legislation that would add the state to the list.

In the fall, the District’s public school system became among the first in the country to allow families to select “non-binary” on their child’s enrollment forms.

But in other states, similar efforts have been met with controversy and resistance. Earlier this month, Indiana’s Bureau of Motor Vehicles announced that it would have a third gender option for driver’s licenses. But a week later, state lawmakers voted to make it more difficult for Indiana residents to change a state-issued ID, requiring not just a physician’s note but also an amended birth certificate that corresponds to the person’s new gender, the Indianapolis Star reported.