Yalow, Rosalyn Sussman (1921-...), an American medical physicist, shared the 1977 Nobel Prize in physiology or medicine with American scientists Roger Guillemin and Andrew Schally . Yalow was recognized for her work in the development of a highly sensitive radioimmunoassay (RIA). An RIA "tags" substances in the blood with a radioactive label, so that even minute quantities can be detected and measured. Yalow and Solomon Berson first used this instrument in 1959 to study insulin concentration in the blood of diabetics. See Guillemin, Roger Charles Louis; Schally, Andrew Victor.