Robin Roberts

Good Morning America anchor Robin Roberts, who beat breast cancer five years ago, has been diagnosed with a rare blood disorder and will undergo a bone marrow transplant.

An emotional Roberts announced on Monday's GMA that she has myelodysplastic syndrome (MDS), a blood and bone marrow disease once known as preleukemia. The disease is likely a result of her breast cancer treatment. Roberts starts pre-treatment Monday in advance of her bone marrow transplant later this year. Her sister — a "virtually perfect match" — will be her donor.

In a message on the GMA website Monday, Roberts wrote that she received her diagnosis "on the very day that Good Morning America finally beat the Today show for the first time in 16 years."

Ratings: Good Morning America ends Today's 16-year win streak

"Talk about your highs and lows!" she wrote. "Then a few weeks ago, during a rather unpleasant procedure to extract bone marrow for testing, I received word that I would interview President Obama the next day. The combination of landing the biggest interview of my career and having a drill in my back reminds me that God only gives us what we can handle and that it helps to have a good sense of humor when we run smack into the absurdity of life."

Roberts will miss a "chunk of time" after the transplant, but she expects things to be "business as usual" at GMA until then.

Though there are "scary" search results about MDS, Roberts said her doctors have assured her that the statistics don't apply to her. "They say I'm younger and fitter than most people who confront this disease and will be cured," she wrote.

"I am going to beat this," she said on air.

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