Sarah Michelle Gellar has likened the loss of Michelle Trachtenberg to the loss of a sister.

On Thursday, Gellar paid tribute to her “Buffy the Vampire Slayer” co-star with a poignant line from their hit TV series, in which the two played siblings.

Trachtenberg — who was best known for her roles in “Buffy” as well as “Gossip Girl,” “Harriet the Spy” and “Ice Princess” — was found dead in her New York City apartment on Wednesday. She was 39.

“‘Michelle, listen to me. Listen. I love you,’” Gellar wrote in a post shared to her Instagram account alongside a slideshow of photos of the two throughout the years. “‘I will always love you. The hardest thing in this world, is to live in it. I will be brave. I will live… for you’ ????????????????️”

Gellar’s quote is a reference to a pivotal scene in the Season 5 finale of “Buffy.”

In the scene, Dawn (Trachtenberg) learns that her blood is the “key” to opening up portals to other dimensions (which is why Gellar included a key emoji in her tribute). When Dawn’s blood is spilled by the season’s villain, several portals from other dimensions open, causing terrifying creatures to inhabit Earth. Amid the chaos, Dawn comes to the bleak realization that the only way to stop this apocalyptic threat is to make her blood stop flowing by sacrificing her own life.

When she tells her older sister, Buffy (Gellar), of her plans, Buffy realizes that she shares the same blood as her sister and decides to sacrifice her own life to spare Dawn.

Before Buffy leaps to her death, she imparts a few lines of wisdom to her teenage sister, which act as a reminder to Dawn that although life can be difficult, Buffy wants her to fearlessly embrace every aspect of it.

“Dawn, listen to me. Listen. I love you,” Buffy tells her younger sister. “I will always love you.”

She then tells Dawn that she’s at peace with her decision and that Dawn needs to remain strong for their loved ones.

“Dawn, the hardest thing in this world, is to live in it,” Buffy says. “Be brave. Live. For me.”

In Gellar’s slideshow of photos of her and Trachtenberg was also an image of the two sitting with the late Shannen Doherty at a Vanity Fair event in 2005.

Doherty, who was close friends with Gellar, died from Stage 4 cancer last July after living with the disease since 2015. She was 53.

After Doherty’s death, Gellar posted a collection of photos of her and the “Beverly Hills: 90210” alum hanging out throughout the years, much like the tribute she shared for Trachtenberg.

“How do you possibly find the right words to sum up 30 years of friendship?” Gellar wrote of Doherty in her post. “I keep reminding myself it only hurts this much because, there was so much love.”