Robin Roberts with her sister Sally-Ann Roberts on “Good Morning America” Monday, June 11, 2012, after Robin Roberts announced she has been diagnosed with myelodysplastic syndrome, a blood and bone marrow disease once known as preleukemia.

*Robin Roberts updated her fans on Friday about her fight with bone marrow disease.

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The “Good Morning America” host, who announced in June that she had contracted the rare disease MDS, had to take the day off to accompany her sister Sally-Ann to the doctor. Sally-Ann is going to be Roberts’ bone marrow donor, having been called a “perfect match.”

Roberts wrote on Facebook:

My sister, Sally-Ann, went through a lot of tests today in preparation for being my bone marrow donor. She was such a trooper. A long day and she had a smile on her face the entire time. I, on the other hand, couldn’t stop crying. I’m so moved by her unrelentless willingness to do whatever it takes to help me.

She also posted the following on Twitter:  As I sit here at the hospital waiting for my sister to complete another test..I want to thank ALL donors. You are angels. Blessings.

As previously reported, Roberts’ revelation of her diagnosis has caused the number of people registering to be bone marrow donors to jump sharply.