Cleto Escobedo, Jimmy Kimmel's band leader, has died at the age of 59.

The musician, who had appeared on Jimmy Kimmel Live! since its 2003 launch episode, was remembered by his high-profile colleague in a heartfelt Instagram post.

"Early this morning, we lost a great friend, father, son, musician and man, my longtime bandleader Cleto Escobedo III," Jimmy, 57, wrote in a caption accompanying a shot of Cleto on stage with a saxophone. "To say that we are heartbroken is an understatement."

He went on to explain he and Cleto had been close friends since they were children.

"Cleto and I have been inseparable since I was nine years old," the late-night host continued.

"The fact that we got to work together every day is a dream neither of us could ever have imagined would come true."

"Cherish your friends," Jimmy added, "and please keep Cleto's wife, children and parents in your prayers."

In 2015, Jimmy told Eyewitness News he had been "nervous' when he first told TV executives he wanted to include his childhood best friend as band leader for his talk show.

"I was nervous, because I thought they'd say, 'We don't want your friend to be the band leader,'" he told the outlet. "So I took the president of ABC to see him play with his band, and he loved it."

He explained it was simply that he knew Cleto would be the best person for the job.

"Of course I wanted great musicians, but I wanted somebody I had chemistry with," Jimmy said. "And there's nobody in my life I have better chemistry with than him."