Marvin Isley, the bass backbone of his sibling band The Isley Brothers, died Monday in Chicago at age 56.

The cause of death has not yet been announced, though Isley suffered from diabetes severe enough to have caused him to leave the band in 1997. His condition led to the eventual amputation of both legs.

Isley’s distinguished bass is a standout on the Isley Bros hits “Fight The Power,” and was also memorable in “Who’s That Lady” [scroll down to watch performance], “For The Love Of You” and “Harvest For The World.” Isley, who grew up in Englewood, New Jersey, wasn’t old enough to join the first incarnation of the Isley Brothers, who have a history snaking back to the mid-’50s and who scored hits in the ’60s like 1966’s “This Old Heart Of Mine” and the funky, 1969 track “It’s Your Thing.”

By the late ’60s, while still of high school age, Isley formed a trio with older brother Ernie and brother-in-law Chris Jasper. By the dawn of the ’70s, those three pacted with the other members of the group to create the classic “3+3” album, which went Top Ten in 1973.

In 1984, the Isleys fractured again. The original group continued to perform under their brand name while Marvin, Ernie, and Chris became Isley-Jasper-Isley. With that group, Marvin scored a No. 1 R&B single with “Caravan of Love.”

For the ’90s, Marvin once again became an Isley Brother. But by ’97s, his illness forced him to quit. In 1992, he was inducted, along with the other key Isley Brothers, into the Rock ‘n Roll Hall of Fame.

Two early members of the group have also died: O’Kelly Isley Jr. in 1996 at age 48 and Vernon Isley, who died in 1955, at the age of just 13, after being hit by a car on his bicycle.

The group’s two best known members – singer Ron Isley and guitarist Ernie Isley – continue to perform under the group’s name.