Founding father of FACS: Professor Leonard A. Herzenberg (1931–2013).

The stunning ability to collect quantitative single-cell analyses of up to 20 parameters simultaneously (1), at rates of tens of thousands of cells per second, and then to divert and recover a specific single live cell, or a subpopulation of live cells, with desired characteristics is the genius of fluorescence-activated cell sorting (FACS). FACS applied in conjunction with fluorescently labeled monoclonal antibodies (Mabs) derived from immortalized hybridoma cell lines is a ubiquitous core technology in medicine and biomedical research, impacting all of immunology, cancer diagnosis, stem cell identification and transplantation, AIDS monitoring, and a variety of therapies, as well as plant and microbial biology. It is, in fact, very difficult to imagine the biomedical research landscape of the last 40 years without FACS. The father of FACS, Leonard “Len” A. Herzenberg of the Department of Genetics at the Stanford University School of Medicine, died on Sunday, October 27, 2013 at the age of 81. Len will be remembered as a deeply innovative scientist, a biotechnology founding figure, a life-long mentor and gentle teacher, an early and effective proponent of diversity in science, and a committed social philanthropist. He is survived by his wife, Leonore “Lee” A. Herzenberg, also of the Stanford Department of Genetics, their children Berry, Jana, John, and Rick, four grandchildren, and an international extended family of former students and postdocs whose lives and careers he and Lee touched and enriched. Len and Lee were a unique team, their partnership transcending essentially all of the humdrum drudgeries of laboratory management and academic piffle for more than five decades. Len’s contributions were all shared with Lee, and they were each other’s staunchest champions and toughest insider critics (2). FACS, in conjunction with fluorochromelabeled Mabs detecting cell surface and intracellular antigens, is most often used to classify normal and neoplastic cells to distinguish functionally distinct subpopulations of immune system cell types, to identify alterations that occur in blood as immunological dysfunction progresses, and to quickly monitor changes during a variety of therapies. FACS can also be used to select specific cells internally expressing fluorescent proteins as a measure of gene activity over developmental and differentiation time frames. Perhaps surprisingly, and certainly less well known, are the important contributions made by FACS in the typing and analysis of bacteria associated with humans, animals, and plants and in understanding microbial diversity in environmental samples isolated from lakes, oceans, and mines. It is difficult to find an area of basic or applied biological research in which cell sorting and analysis with flow cytometry instruments is not a key technology. Not surprisingly, flow cytometry instruments are now produced by a variety of manufacturers, and tens of thousands of machines are currently in use around the world, making this technology a cornerstone of all of modern biology. Len was born November 5, 1931, in Flatbush, Brooklyn, where he grew up, attended Brooklyn College, and met his future wife. Len fled to Pasadena and Cal Tech, where he received his PhD, and after a short period of separation and long-distance phone bills, he and Lee married. Lee became, in essence, the first female to earn an undergraduate degree at Cal Tech, acknowledged by signed letters from her various course instructors detailing her stellar academic performance. Len and Lee were then off to Paris, where Len was an American Cancer Society postdoc with Jacques Monod, studying the induction of Lac permease. After a short stint at the National Institutes of Health, Len was lured to Stanford as a founding member in Joshua Lederberg’s new Department of Genetics in the newly opened Medical School, recently decamped from San Francisco to Palo Alto. Len never left, and his papers from that earliest era show a decided interest in the genetic control of immune cell function. Len quickly recognized the need for high-throughput, quantitative fluorescence measurements in the early 1960s. Frustrated with his own poor eyesight and the hours spent staring at cells under the fluorescent microscopes of the day, Len capitalized on advances in flow sorting of cell-sized particles made by Mack Fulwyler at the Los Alamos National Laboratory in conjunction with oscillatory ink-jet printing techniques developed by Richard (Dick) Sweet at the Stanford Electronics laboratory and later at Varian Associates and Xerox. Len seized on the visionary idea to build an instrument capable of identifying and rapidly sorting live cells with distinctive surface molecules detected by fluorescently tagged antibodies. Well before “interdisciplinarity” had achieved its current status as an abused buzzword, Len recognized that disrupting technological boundaries required innovative scientists from a variety of disciplines. He assembled and deployed a team of engineers, eventually including Dick Sweet, physicists, and computer scientists, and put the first FACS into Len and Lee Herzenberg with a FACS, circa 1985.