Klein's passing was first confirmed by Simmons' wife, Shannon Tweed Simmons.
"Rest in peace my dear Flora," Tweed wrote in a social media post. "The best grandma and mother in law I could ask for. You were loved and you will be missed."
Gene himself added that Flóra was "The best mother in the world."
Gene's band mate, Kiss front man Paul Stanley, added his own tribute.
"I knew here as long as I've known Gene," Stanley wrote. "Beyond her fierce love and pride in her only child, she was his inspiration to live up to being worthy of the sacrifices she made in a very difficult and, at times, horrific life."
Stanley continued, noting that he "admired and loved" Flóra, a Holocaust survivor and single mother, who left Israel for Queens, New York, in the 1950s in search of a better life.
"I will miss her laugh and smile," Stanley wrote. "Let us all say a prayer for her, Gene and his family."
Gene and Stanley together spoke earlier this year about Flóra's involvement in Kiss's early days.
Gene recalled at a Vault event this past spring in Las Vegas about how his mother once referred to Stanley as "a bum."
"Yup," Stanley confirmed, before elaborating. "I call his house and I want to talk to him and his mom, bless her heart...She has a thick accent and she thinks the world revolves around him — and that's what he thinks too. I call up and I say, 'Can I speak to Gene?' and she says, 'The king is on the throne.' And I go, 'Yeah, that's nice. Can I speak to Gene?'"
"'The king is on the throne,'" Stanley recalled her repeating. "I go, 'Nice, the king is on the throne. Can I speak to Gene?'"
Gene's mother repeated herself one more time before Stanley got the message: nature had called earlier and Gene was answering, moving his bowels.
Gene has often spoken lovingly over the years about his mother's influence on his life. He credits her with inspiring his work ethic, and with keeping him away from drug and alcohol abuse.
As an only child, Simmons said he would ask himself, in light of his mother's former suffering, "What right do I have to torture her and make her unhappy? No, that's never gonna happen."
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