Stephen J. O'Brien (born September 30, 1944) is an American geneticist. He was the editor-in-chief, and is currently the advisory editor of the scientific journal Journal of Heredity. He received a Ph.D. degree studying genetics at Cornell University in 1971 and was a Cornell A. D. White Professor-at-large from 1999-2005.
He is known for his contributions on the evolution of mammalian viruses and the adaptation to this exposure by the host, but also works on mammalian systematics. He works as chief in the Laboratory of Genomic Diversity, National Cancer Institute, USA. O'Brien is also the author or editor of several books and monographs, including:
Some of his work has been on the discovery and explanation of low genetic variability in the cheetah species. A large contributor to the hypothesis that the cheetah was reduced in population size due to a large bottlenecking event, O’Brien has been analyzing the causes and effects of the cheetah’s genetic homogeny since 1980, and it is one of the most published authors on the subject.[1]