(CNN) Washington state residents who don't identify as male or female will soon be able to choose X as their gender on birth certificates.

Starting January 27, they will be able to identify as male, female or X on birth certificates. The policy lets Washingtonians change existing documents; it does not apply to new ones.

The rule, announced Thursday by the state Department of Health , defines X as the following:

A gender that is not exclusively male or female, including, but not limited to, intersex, agender, amalgagender, androgynous, bigender, demigender, female-to-male, genderfluid, genderqueer, male-to-female, neutrois, nonbinary, pangender, third sex, transgender, transsexual, Two Spirit, and unspecified.

For adults seeking to change their birth certificate from male to female or vice versa, the rule removes an existing requirement of a doctor's letter. Minors who want to change their designation must have written consent of their parent or legal guardian and documentation from a licensed health care professional.