Gunnar Peterson's family is facing a heartbreaking health journey.

The celeb fitness trainer shared that his and Jess Peterson's 4-year-old daughter Monroe Vivian Peterson was recently diagnosed with acute myeloid leukemia, a blood and bone marrow cancer.

As Gunnar explained in a March 26 Instagram message, Monore "had a nagging cough, on and off fever, and an odd pallor to her for a few days last week. Typical kid stuff. Then my wife noticed bruising on her legs which was odd since she hadn't been very active because of the other symptoms, and she took her to the hospital."

Four days later, her health had rapidly changed. "White blood cell count KO'd. Platelets in the toilet," Gunnar noted. "Hemoglobin number lower than a soccer game final. Hematocrit front row on the struggle bus. Needs immediate transfusion(s). Has to have bone marrow biopsy. Flow cytometry. Spinal tap(s). Chemotherapy. And finally, Make-A-Wish candidate."

"How the f*#! did we get here?!?!" he continued. "More to come as we navigate the nightmare… Our Warrior Princess will crush this! She has even said it herself! Please hold our daughter, and our family-immediate and extended-in your hearts, thoughts, and prayers."

After sharing his daughter's diagnosis, Gunnar received a flood of well-wishes from friends, including Kris Jenner. 

"My family and I are praying for your baby G," she commented, "we love you all so very much and lifting her up in the name of Jesus."

Kim Kardashian also wrote to Gunnar, "We are all praying for you [heart emoji] we love you."

Meanwhile, Dwayne Johnson shared a moving message to the Peterson family.

"We're always here brother. Always," he commented. "She will crush this, as she herself has declared."