Good Morning America cohost Robin Roberts, who five years ago battled breast cancer, on Monday teared up on the show as she revealed that she has the rare blood and marrow disorder myelodysplastic syndrome. Roberts, 51, said she had begun chemotherapy and will need to get a bone-marrow transplant this fall. She has a perfect match: her sister Sally Ann Roberts, a TV anchor in New Orleans. "My doctors tell me I'm going to beat this, and I know it's true," said Roberts, who developed MDS as a result of her cancer treatments, an extremely rare form of transmission. Roberts said that in a cosmic irony, she had her bone marrow extracted for testing the same day she landed a coveted interview with President Obama.