Robin Roberts 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