Chris Squire, co-founder and bassist of U.K. progressive rock legends Yes, died on Saturday after a struggle with leukemia. He was 67.

The gifted musician and songwriter was the one constant in the band’s many lineups; his complex, muscular, melodic bass lines and supple backing vocals were essential to the Yes sound.

A message from the band on its Facebook page reads:

It's with the heaviest of hearts and unbearable sadness that we must inform you of the passing of our dear friend and Yes co-founder, Chris Squire. Chris peacefully passed away last night in Phoenix, Arizona, in the arms of his loving wife, Scotty.

In addition to being an architect of Yes’ albums—including classics The Yes Album, Fragile, Close to the Edge and Going for the One and the hit '80s set 90125—Squire crafted the acclaimed 1975 solo album Fish Out of Water.

Squire and singer Jon Anderson formed the group in London in 1968. In the dawning years of the subsequent decade, Yes helped define and stretch the possibilities of the nascent progressive-rock form. Their songs, often whole album sides in length, merged hard-charging rock, tricky tempo changes, orchestral sweep and folky beauty. Squire’s intricate bass work drove both melody and groove, and his solos had the compositional brio of mini-sonatas.

The band toured steadily in the ensuing decades, despite frequent changes in its membership and in the musical fashion. A hiatus was announced in May when Squire’s illness was made public.

In addition to Scotty, Squire is survived by his five children.

A statement from former Atlantic chief Jerry Greenberg reads: “I had the honor to work with Chris Squire and Yes during those great years as the President of Atlantic Records. Chris was the foundation for Yes as most bass players are in rock bands. He founded the group with Jon Anderson and played on every Yes record. We spoke only a few months ago and he was very happy he was about to tour again. His music and friendship I will always remember.”

Added NARAS chief Neil Portnow, the Grammy winner "was an exceptional bass player and talented vocalist. From his humble beginnings as a church choirboy in London, to selling out arenas worldwide with Yes, it was his commitment to mastering his craft, as well as his enthusiastic demeanor, that made him one of the most respected and influential bass players of all time. We have lost an undeniably gifted musician and our heartfelt condolences go out to Chris’ family and friends, and to all of those he entertained during his impressive career."

The prog fans in this office salute his musical genius and add our own condolences to his family and bandmates.