Actress and New York gubernatorial candidate Cynthia Nixon has announced that her oldest child has come out as transgender.

The former “Sex and the City” star posted the announcement on her Instagram account on Friday to mark the 14th annual Trans Day of Action. Her 21-year-old son Samuel Joseph Mozes, who goes by the nickname Seph, originally was named Samantha.

Seph is the child of Nixon and former partner Danny Mozes. Nixon also has a 15-year-old son with Mozes and a 7-year-old son with her wife, Christine Marinoni.

Nixon accompanied Friday’s announcement with an Instagram photo of herself with her son in his cap and gown at his graduation earlier this month from the University of Chicago.

Nixon is challenging Gov. Andrew Cuomo in New York’s Democratic primary for governor.