Shim Jaehyun, the K-pop star known mononymously as Jaehyun, has died at age 23 after a battle with leukemia. He was best known for being a part of the band F.ABLE.

News of his death spread this week after an announcement on June 29. Shortly afterward, Jaehyun’s former F.ABLE bandmate Hojun posted a tribute to the late star, writing on Instagram Stories, “I heard the news too late … I’m so sorry I couldn’t be there for your last journey.”

“I sometimes think about how I could have done better, and it’s regretful,” Hojun continued. “I hope you can shake off all your worries and live comfortably there. Thank you for the roughly 5 years. I will pray that you eat a lot of delicious food and do well. Come visit me in my dreams.”

Jaehyun reportedly kept his illness private leading up to his death.

The Mayo Clinic defines leukemia as a cancer that affects a person’s blood-forming tissues, such as their bone marrow and lymphatic system. “Leukemia usually involves the white blood cells,” the organization explains. “Your white blood cells are potent infection fighters — they normally grow and divide in an orderly way, as your body needs them. But in people with leukemia, the bone marrow produces an excessive amount of abnormal white blood cells, which don’t function properly.”

F.ABLE debuted in 2020 under Haeirum Entertainment, releasing first single “Burn It Up” that year. After dropping additional singles “Run Run Run” and “Baby Don’t Cry,” the band went inactive, and three of the members eventually formed a new group called ENPHAZE, while Jaehyun stepped back from the public eye, according to All K-Pop.

But even though Jaehyun wasn’t an active performer in K-pop in recent years, many fans online are remembering him in light of the news of his death. “This hurts really bad,” one fan wrote on X. “I was with the group for a short period of time and wondered what happened to him … His suffering is now over.”

Another person wrote, “He was really special and important to me.”