Immunologist and geneticist Ray D. Owen, professor of biology, emeritus, at Caltech passed away on September 21. Owen's most significant contribution to immunology was his 1945 discovery of immunological tolerance in fraternal twin cattle. Using blood typing, he discovered that each twin had no immune response to the foreign antigens introduced from the other twin. His findings paved the way for the experimental induction of tolerance through immune suppression and for early tissue grafting—which initiated the era of organ transplantation. Owen's later work included studies on human antibodies, blood-group antigens, the evolution of immune systems, and the genetic analysis of the major histocompatibility complex—a large family of genes that plays an important role in the immune system and autoimmunity. Dr. Owen received his PhD in genetics from the University of Wisconsin and became an associate professor at Caltech in 1947; where he was promoted to full professor in 1953 and became professor emeritus in 1983. Not only was Owen a brilliant scientist, he was highly recognized for his extraordinary dedication to mentorship at Caltech, where he launched the effort to admit female undergraduate students; which ultimately allowed the first female undergraduates to enroll at the Institute in 1970. His accomplishments led Leonore Herzenberg, professor of genetics at the Stanford School of Medicine and pioneer of immunology, to write "Dr. Owen's belief in the genderlessness and color-blindness of intelligence and creativity has encouraged men and women to excel in their chosen fields," in a letter recommending Owen for a lifetime mentoring award.