Gary Sinise is mourning the death of his son, McCanna Anthony “Mac” Sinise.

The “Forrest Gump” actor announced the news on his Gary Sinise Foundation website Tuesday and explained that Mac was diagnosed with chordoma on Aug. 8, 2018. The actor’s only son died from the “very rare cancer” on Jan. 5. He was 33.

“Like any family experiencing such a loss, we are heartbroken and have been managing as best we can,” Sinise wrote in his solemn tribute. “My heart goes out to all who have suffered a similar loss, and to anyone who has lost a loved one.”

“We’ve all experienced it in some way,” he continued. “Over the years I have met so many families of our fallen heroes. It’s heartbreaking, and it’s just damn hard. Our family’s cancer fight lasted for 5 1/2 years, and it became more and more challenging as time went on.”

Sinise wrote in his announcement that Mac was diagnosed mere months after his wife, Moira Harris, was diagnosed with stage 3 breast cancer. While she went into remission post-treatment, Mac’s cancer continued to spread and “disabled him more and more as time went on.”

According to the Mayo Clinic, chordoma is a rare bone cancer most commonly located in the skull or spine. It’s most often diagnosed in adults between 40 and 60, but can occur at any age. While it grows slowly, its proximity to such valuable areas makes it difficult to treat.

Mac lived with chordoma for nearly six years before the disease ended his life.

His father said the “exceptional drummer” and University of Southern California graduate, who studied songwriting and composition, had only recently joined his foundation when he was diagnosed. Founded in 2011, the charity raises money for wounded veterans.

Mac underwent five separate surgeries on his spine between 2018 and 2020, and eventually became paralyzed from the chest down due to the cancer, but nonetheless forged ahead with his album, “Resurrection & Revival.” The album went to press the week Mac died, and proceeds from sales are purportedly going to the Gary Sinise Foundation.

“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,” Sinise wrote on his website. “He fought an uphill battle against a cancer that has no cure, but he never quit trying.”

Sinise remains best known for his role as Lieutenant Dan, and frequently performed with Mac and his Gary Sinise & Lt. Dan Taylor Band. The grieving father is not alone, however, as Mac is also survived by his mom, sisters, grandmother, aunts, uncles and cousins.

“I am so blessed, fortunate, and proud to be his dad,” Sinise wrote Tuesday.