Simon Flexner (March 25, 1863 in Louisville, Kentucky – May 2, 1946) was a physician, scientist, administrator, and professor of experimental pathology at the University of Pennsylvania (1899–1903). He served as the first director of the Rockefeller Institute for Medical Research (1901–1935) (later developed as Rockefeller University) and a trustee of the Rockefeller Foundation. He was also a friend and adviser to John D. Rockefeller Jr. Among Flexner's most important achievements are studies into poliomyelitis and the development of serum treatment for meningitis. Among his lab assistants were Hideyo Noguchi and Cornelius Rhoads, later directors of Memorial Hospital and the Sloan-Kettering Institute, respectively. The bacteria species Shigella flexneri was named in recognition of Flexner. In addition, Flexner was the first to describe Flexner-Wintersteiner rosettes, a characteristic finding in retinoblastoma, a type of cancer. Early life and career Simon was born in Louisville, Kentucky, to Moritz (Morris) Flexner, an immigrant from Neumark, Bohemia, via several years in Strasbourg, France; and Esther from Roden, Germany. He was the fourth son of seven in a large family of nine children: Jacob Flexner, Henry, and Isadore; then Simon, followed by Bernard Flexner, Abraham Flexner, and Washington. The two sisters Mary and Gertrude were the youngest. Jacob became a pharmacist and physician; Bernard became a Zionist leader, and Abraham became an educator, eventually influencing the direction of medical education in the United States. Simon first gained a degree from the Louisville College of Pharmacy and worked with his brother Jacob for eight years. Medical school and career He returned to college, getting his medical degree from Louisville Medical College in 1889. He did postgraduate work in pathology at Johns Hopkins University Medical School, and started teaching there. By 1899, he was a professor of pathology at the University of Pennsylvania. Flexner was elected to the American Philosophical Society in 1901. He taught at Penn until 1903, but was called to the Rockefeller Institute for Medical Research (later Rockefeller University), where he started serving as its first director in 1901. He managed the research institute until 1935. Through this affiliation and related work, he came to know the philanthropist John D. Rockefeller, who supported research and basic medical care. In December 1907 Flexner declared in a reading of his paper on "Tendencies in Pathology" in the University of Chicago that it would be possible in the then-future for diseased human organs substitution for healthy ones by