The resultant radiolabeled sdAb conjugate was directly compared with that labeled using [*I]IB-Mal-d-GEEEK in vitro in HER2-expressing BT474M1 human breast carcinoma cells and in vivo in athymic mice bearing HER2-expressing SKOV-3 human ovarian carcinoma xenografts. The gene discussed is ERBB2; the disease is ovarian carcinoma.