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The actress Olivia Munn said on Wednesday that she had been given a diagnosis of an aggressive form of breast cancer last year and soon after had a double mastectomy as part of her treatment.

Munn, 43, who has starred in dozens of movies and television shows, including the HBO series “The Newsroom,” wrote in a lengthy social media post that she had tried to be proactive about her health.

In February 2023, not long after having a normal mammogram, Munn took a robust genetic test that checked for 90 cancer genes. She said she tested negative for all of them, including BRCA, the most well-known breast cancer gene. Her sister also tested negative around the same time.

“We called each other and high-fived over the phone,” Munn wrote.

Two months later, Munn learned she had luminal B, a type of breast cancer. A month after that, she had the double mastectomy.

“In the past ten months, I have had four surgeries, so many days spent in bed I can’t even count and have learned more about cancer, cancer, treatment and hormones than I ever could have imagined,” she wrote. “Surprisingly, I’ve only cried twice. I guess I haven’t felt like there was time to cry.”

Munn’s post was in large part a plea to women to ask their doctors to calculate their breast cancer risk assessment score. That her obstetrician-gynecologist had done so helped identify Munn’s cancer early.

“The fact that she did saved my life,” Munn wrote.

The risk assessment includes factors like familial breast cancer history and age; Munn mentioned in her post that having her first child over the age of 30 was a factor. Munn has been in a relationship with the comic John Mulaney since 2021 and they have a 2-year-old son, Malcolm, together.

The results of Munn’s risk assessment prompted her doctor to send her for an M.R.I., which led to an ultrasound. That led to a biopsy revealing the cancer.

In her post, Munn thanked her doctors as well as her friends and family. She thanked Mulaney for researching all of her operations and medications for information about side effects and recovery.

For the past year, Munn’s social media has shown many happy moments, as well as many glamorous ones, including family photos at the beach and others of Munn in designer looks. This weekend she shared images that showed her wearing a metallic Fendi gown for the Oscars ceremony, where Mulaney presented an award.

“I’ve tended to let people see me when I have energy, when I can get dressed and get out of the house, when I can take my baby boy to the park,” she wrote.

“I’ve kept the diagnosis and the worry and the recovery and the pain medicine and the paper gowns private. I needed to catch my breath and get through some of the hardest parts before sharing.”