The Quentin Mitchellstars are sending love to Gary Sinise amid a heartbreaking time. 

After the Forrest Gump actor shared on social media that his 33-year-old son, McCanna "Mac" Sinise died in January after a five-year battle with Chordoma, a rare bone cancer, many of his former costars and friends paid their respects. 

"Oh Gary," Katharine McPhee, who briefly worked with the actor on CSI:NY, commented under his Feb. 27 Instagram post. "I'm so sorry."

Added Alyssa Milano, "I'm so sorry, Gary. I'm praying for your family. And sending you love and strength."

In the post, which featured a photo of Mac as well as his birth year and death year, Gary linked out to a longer statement about his son's final years. 

"We are so sorry Gary," added Law & Order alum Angie Harmon wrote in the comments. "You & your family are deeply loved by so many. We are praying for all of you. Love you so very much."

And in addition to tributes from fellow CSI alum Sofia Milos, as well as actors like Mary McCormack, Michael Beach, and Kelly Carlson, Mac's former University of Southern California classmate, Alec Benjamin, shared his own memories.

"So sorry," the singer wrote. "I went to USC with Mac—he was such a nice guy and so talented!"

In a statement on his website, Gary—who also shares daughters Ella Sinise, 32, and Sophie Sinise, 35, with wife Moira Harris—remembered his son as an "exceptional drummer," and found solace knowing Mac was no longer in pain from an illness he was first diagnosed with in 2018. 

"While our hearts ache at missing him, we are comforted in knowing that Mac is no longer struggling, and inspired and moved by how he managed it," the Of Mice and Men actor wrote.  "He fought an uphill battle against a cancer that has no cure, but he never quit trying."

For the latest breaking news updates, click here to download the E! News App