Cynthia Nixon saluted her son Samuel today as the pair celebrate the 14th Annual Trans Day of Action.

The proud mom and former Sex and the City star, who is currently running for Governor of New York, took to Instagram to post a picture of the pair and openly share for the first time that 21-year-old Samuel, born Samantha, is transgender.

'I'm so proud of my son Samuel Joseph Moses (called Seph) who graduated college this month,' she wrote. 'I salute him and everyone else marking today’s #TransDayofAction. #TDOA.'

Congrats: Cynthia Nixon shared warm words for her son Samuel, called Seph, on Instagram today while celebrating the 14th Annual Trans Day of Action

Transition: Samuel was born Samantha in 1996 but Cynthia explained in her Instagram caption that he now goes by 'Seph'

Proud mom: Cynthia is pictured with wife Christine and kids Charles (top), who is now 15, and Seph (bottom right), who is now 21, in June 2006

In the image, the two are seen with their arms around one another, smiling at the camera at Seph's graduation from The University of Chicago; Samuel is dressed in a cap and gown.

Seph was born Samantha Mozes in November 1996 to Cynthia and her then-partner and college sweetheart Danny Mozes.

Cynthia and Danny were together from 1988 to 2003, and also had another son, Charles Ezekiel, who was born in December of 2002.

Shortly after their split in 2004, Cynthia began dating Christine Marinoni, an activist who, until recently, worked for New York City's Deputy Mayor Richard Buery. Cynthia and Christine married in May 2012. They have another son together, Max, whom Christine gave birth to.

Seph has been mostly out of the public eye lately. Though he was occasionally spotted by paparazzi while out with his mom as a little kid, he's been keeping a low profile since Cynthia announced her bid for governor against incumbent Andrew Cuomo.

Cynthia's attended several Pride events this month and in years past, but Seph hasn't yet tagged along or appeared at any campaign stops for his mom.

Speaking out: Seph, pictured in recent Facbeook images, seems to have been living as a trans man for some time, but this is the first time that Cynthia has addressed it publicly in any way

Growing up: Cynthia is pictured with Seph in New York City in August 2014

Flashback: Cynthia and and Seph are pictured with actress Rosario Dawson in January 2010

Yet Cynthia has never really spoken much about any of her kids, keeping her personal life mostly private.

Over a decade ago, she told the New York Post what she thought it might be like for her kids at school having a famous mother.

'In a school where everyone is famous or rich or whatever, you have a culture. "What does your dad do?" "What does your mom do?" she said. 'Of course, I’m not saying my daughter doesn’t feel a little like a freak because she has a famous mom.

'At school this morning, she was like, "Go, just go, just go." Which is a kid thing anyway, but you know — I attract attention. When you’re 9, you don’t want that.'

It does seem that Seph has been living as trans for several years now, though this is only the first time that that Cynthia has publicly acknowledged it.

Cynthia has, however, gone to bat for Seph and other trans individuals during her campaign and in her role as LGBTQ advocate.

Ex: Cynthia and Seph's dad, Danny Mozes (pictured together in May 2000), were together from 1988 to 2003 and had two children

Happy: Cynthia married Christine Marinoni, an activist who, until recently, worked for New York City's Deputy Mayor Richard Buery, in May 2012

Family: Cynthia is pictured with Danny (left), wife Christine (second from right), and their youngest son Max, in an Instagram photo shared on Father's Day earlier this month

'Happy #TransDayofVisibility to all the adults, teens, & children in the US who identify as transgender,' she wrote on Twitter on March 31 of this year.

'We see you. We recognize you. And now more than ever in this Trump discrimination administration, we pledge to fight for you and for your rights. Trans rights are human rights.'

Last year, she spoke about the importance of LGBTQ coming out and standing up for rights.

'As LGBT people, we know how important coming out is, but I would argue that our coming out has never been more important than it is right now,' she said at a rally.

'We need to come out not just as queer, but as people who know all too well what it feels like to be put in a box that says "other," less than," "easy target if you’re looking for someone to bully, harass, discriminate against, demonize, beat up, even kill,"' she said.

In the running: Cynthia, pictured earlier this month, is currently challenging incumbent Andrew Cuomo for the democratic nomination for Governor of New York

Kids: Cynthia and Danny had another son, Charles (far right), and she and Christine had a son named Max (left), who Christine gave birth to in 2011

Pride: Cynthia has also marched in New York City Pride parades, including in 2016 with Mayor Bill de Blasio, his wife Chirlane McCray, and Al Sharpton (as well as her wife and son Max)

Meanwhile, Cynthia's wife Christine has also spoken out about the need for more help for the trans community.

'I think there’s a lot of work to be done on behalf of trans folks because they're facing some of the same issues that we've been able to overcome on some level,' she told City & State New York in 2017.

'I think we’ve come a really long way; I think we also have a far way to go. We’re going to have to wade through the current backlash to make sure we don't lose some of what we’ve gained.

Seph seems to recognize that her parents are standing up for her. In 2002, Cynthia was arrested outside City Hall in Lower Manhattan during a protest, in which she and others were demanding better funding for public school.

According to People, Cynthia warned Seph at the time that she might be arrested for the protest and was impressed by the response.

'My daughter herself brought up Martin Luther King,' she told Newsday. 'And so she says she understands that sometimes you have to go to jail to make things better.'