Ann Romney to Robin Roberts: 'Keep on fighting!'

Ann Romney tweeted encouragement to Robin Roberts on Thursday from the Republican National Convention.

Thursday was the ABC anchor's final appearance on "Good Morning America" before starting medical leave for a bone marrow transplant.

She promised to return "as soon as I can."

Roberts has linked her diagnosis of myelodysplastic syndrome to her previous battle with breast cancer. Ann Romney is a breast cancer survivor and called to encourage Roberts following her announcement in June.