On August 30, 2012, Roberts went on medical leave after undergoing a bone marrow transplant (donated by her sister and fellow news anchor SallyAnn Roberts) following her diagnosis with myelodysplastic syndrome. "GMA" correspondent Amy Robach and "20/20" anchor Elizabeth Vargas served as the primary substitutes, typically alternating every other week. Others, including some celebrities, also served as special guest anchors during this time such as Kelly Ripa, Jessica Simpson, Barbara Walters and Elisabeth Hasselbeck. On January 14, 2013, Roberts announced that she hoped to return to the program sometime that February; Roberts performed dry run rehearsals during the week of January 21 in preparation for her return; she and her doctors evaluated her body's reaction to the makeup, the lighting, her hair, and the number of people she comes in contact with to avoid compromising her then-gradually-improving health. Roberts returned to "Good Morning America" part-time on February 20, 2013; she announced in August 2013 that she would return to full-time hosting duties on September 3 of that year.