To the Editor.
—This letter concerns our laboratory's research on Kaposi's sarcoma (KS), the current state of the art, and our response to criticisms in the article by Dr Witte and colleagues1 regarding our publication in Science.2As those familiar with the field are aware, KS is a unique vascular tumor that occurs frequently in human immunodeficiency virus (HIV)—infected patients. The disease remains relatively difficult to treat and is associated with significant morbidity, particularly in its pulmonary, gastrointestinal, and disseminated forms. In 1986, initially looking for an associated virus, we were able to grow cells from KS patients that we believe are vascular endothelial in origin and play a role in establishing the tumor. Using these cells, we have developed the hypothesis that cytokines and the HIV regulatory protein Tat promote the growth of KS cells. The fact that these KS cells are not unique to KS lesions