Earlier, “Good Morning America” anchor Robin Roberts tweeted that she planned to announce personal news.

Moments ago, she told her audience she has M.D.S., or myelodysplastic syndrome.

Roberts said:

I beat breast cancer. … Sometimes treatment for cancer can lead to other serious medical issues. That’s what I’m facing right now. MDS – myelodysplastic syndrome. It is a rare blood disorder that affects the bone marrow.

The 51-year-old begins treatment today and expects to undergo a bone marrow transplant. Her sister will be her donor.

The Twitterverse erupted in well wishes and supportive tweets.

Roberts shares her thoughts about the diagnosis in an open letter.

My doctors tell me I’m going to beat this — and I know it’s true.

If you Google MDS, you may find some scary stuff, including statistics that my doctors insist don’t apply to me.  They say I’m younger and fitter than most people who confront this disease and will be cured.

Today, I will start what is known as pre-treatment -– chemotherapy in advance of a bone marrow transplant later this year. Bone marrow donors are scarce and particularly for African-American women.  I am very fortunate to have a sister who is an excellent match, and this greatly improves my chances for a cure.  As you know from my recent interview with Mark Zuckerberg, organ donation is vitally important. Many people don’t realize they can be bone marrow donors.  I encourage everyone to sign up on a donor registry like bethematch.org.

I received my MDS diagnosis on the very day that Good Morning America finally beat the Today Show for the first time in 16 years.  Talk about your highs and lows!

Roberts expects to work during her illness, but will “miss a chunk of time” when she undergoes the transplant later this year.

The Twitchy team wishes Roberts all the best as she overcomes this hurdle.