07/07/2012 09:53 am ET Updated Sep 06, 2012

Robin Roberts: 'I Couldn't Stop Crying' Watching My Sister Take Medical Tests To Be My Donor

Robin Roberts updated her fans on Friday about the latest round of her fight with bone marrow disease.

The "Good Morning America" host, who announced in June that she had contracted a rare disease called 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 messages on Twitter:

Roberts' revelation of her diagnosis has caused the number of people registering to be bone marrow donors to jump sharply.