'90210' cast pays tribute to ailing Shannen Doherty

Shannen Doherty, left, is shown with her mother, Rosa Elizabeth, on Oct. 7, 2016. (Instagram / Shannen Doherty)

CHICAGO -- "Beverly Hills, 90210" cast members are paying tribute to ailing co-star Shannen Doherty.

Doherty is being treated for breast cancer and didn't take part in a "90210" reunion panel at the Rewind Convention in Chicago over the weekend.

Luke Perry, whose character had an on-screen romance with Doherty's, told the audience Saturday that Doherty was "a very big part of the success of the show." He added that Doherty is "not doing well right now but sometimes her contributions are minimized."

Fellow cast mate Jennie Garth praised Doherty on Instagram by sharing a picture Saturday with a quote, "Fight Like A Brenda," a reference to Doherty's "90210" character, Brenda Walsh. Garth calls Doherty the "strongest lady I've ever known."