Anti-Tac had been discovered by Thomas A. Waldmann, M.D., chief of the Metabolism Branch at the National Cancer Institute and his team, and they had conducted animal studies and a small clinical trial of anti-Tac in people with T-cell leukemia, with promising results, but people quickly developed their own antibodies rejecting the mouse protein; Waldman, and his colleagues then approached Protein Design Labs to humanize the antibody.