Kenneth S. Kendler, MD, Director of the Virginia Institute for Psychiatric and Behavioral Genetics, et al., wrote in the 2000 American Journal of Psychiatry article "Sexual Orientation in a U.S. National Sample of Twin and Non-Twin Sibling Pairs":
"In accord with findings from prior twin studies, resemblance for sexual orientation was greater in monozygotic twins than in dizygotic twins or nontwin sibling pairs. These results suggest that genetic factors may provide an important influence on sexual orientation."
Brian S. Mustanski, PhD, Assistant Professor of Psychiatry at the University of Illinois at Chicago, stated in a Jan. 27, 2005 University of Illinois at Chicago news release about his Human Genetics article "A Genomewide Scan of Male Sexual Orientation":
"There is no one 'gay' gene. Sexual orientation is a complex trait, so it's not surprising that we found several DNA regions involved in its expression.
Our best guess is that multiple genes, potentially interacting with environmental influences, explain differences in sexual orientation.
Our study helps to establish that genes play an important role in determining whether a man is gay or heterosexual."
Ebru Demir, PhD, Researcher at the Institute of Molecular Biotechnology of the Austrian Academy of Sciences, and Barry J. Dickson, PhD, former Scientific Director of the Institute of Molecular Biotechnology of the Austrian Academy of Sciences, wrote in their 2000 Cell article titled "Fruitless Splicing Specifies Male Courtship Behavior in Drosophila":
"We show that male [gene] splicing is essential for male courtship behavior and sexual orientation. More importantly, male [gene] splicing is also sufficient to generate male behavior in otherwise normal females. These females direct their courtship toward other females (or males engineered to produce female pheromones).
The splicing of a single neuronal gene thus specifies essentially all aspects of a complex innate behavior."
Dean H. Hamer, PhD, former Chief of the Section on Gene Structure and Regulation at the Laboratory of Biochemistry at the National Cancer Institute, et al., wrote in the July 16, 1993 Science article "A Linkage Between DNA Markers on the X Chromosome and Male Sexual Orientation":
"The role of genetics in male sexual orientation was investigated by pedigree and linkage analyses on 114 families of homosexual men. Increased rates of same-sex orientation were found in the maternal uncles and male cousins of these subjects, but not in their fathers or paternal relatives, suggesting the possibility of sex-linked transmission in a portion of the population.
DNA linkage analysis of a selected group of 40 families in which there were two gay brothers and no indication of nonmaternal transmission revealed a correlation between homosexual orientation and the inheritance of polymorphic markers on the X chromosome in approximately 64 percent of the sib-pairs tested.
The linkage to markers on Xq28, the subtelomeric region of the long arm of the sex chromosome, had a multipoint lod score of 4.0 (P = 10(-5), indicating a statistical confidence level of more than 99 percent that at least one subtype of male sexual orientation is genetically influenced."
Kenneth M. Cohen, PhD, Lecturer in Human Development at Cornell University, wrote in the 2002 Archives of Sexual Behavior article "Relationships Among Childhood Sex-Atypical Behavior, Spatial Ability, Handedness, and Sexual Orientation in Men":
"Recent scans of the human genome reveal that some gay males share a genetic marker for homosexuality on the X chromosome. One avenue through which genes regulate homoeroticism is by instructing the brain to develop in a sex-atypical manner."
William Byne, MD, PhD, Director of the Laboratory of Neuroanatomy and Morphometrics at Mount Sinai School of Medicine, stated in his 1995 Journal of Homosexuality article "Science and Belief: Psychobiological Research on Sexual Orientation":
"While some authors have speculated about the existence of 'genes for homosexuality,' genes in themselves cannot directly specify any behavior or cognitive schema. Instead, genes direct a particular pattern of RNA synthesis which in turn specifies the production of a particular protein.
There are necessarily many intervening pathways between a gene and a specific behavior and even more intervening variables between a gene and a pattern that involves both thinking and behaving.
The term 'homosexual gene' is, therefore, without meaning, unless one proposes that a particular gene, perhaps through a hormonal mechanism, organizes the brain specifically to support a homosexual orientation."
Jeffrey Satinover, MD, Founder and former Director of the Sterling Institute for Neuropsychiatry and Behavioral Medicine, wrote in his article "The Gay Gene?" published on Leadership U, a Campus Crusade for Christ International website (accessed July 13, 2002):
"There is not any evidence that shows that homosexuality is 'genetic', and none of the research itself claims there is. Only the press and, sadly, certain researchers do-when speaking in sound bites to the public.
Homosexuality may run in families but you get viruses from your parents, too, and some bad habits. Not everything that is familial is innate or genetic."
Simon LeVay, PhD, Neurologist and Co-Founder of the Institute of Gay and Lesbian Education, stated in a 1994 Discover article entitled "Sexual Brain":
"It's important to stress what I didn't find. I did not prove that homosexuality is genetic, or find a genetic cause for being gay. I didn't show that gay men are born that way, the most common mistake people make in interpreting my work. Nor did I locate a gay center in the brain."
George P.A. Rice, PhD, Professor of Neurology at the University of Western Ontario, et al., wrote in the 1999 Science article titled "Male Homosexuality: Absence of a Linkage to Microsatellite Markers Xq28":
"We still contend that an X-linked gay gene could not exist in the population with any sizable frequency, due to the strong selection against it...
We agree with Hamer that our results do not exclude the possibility of genetic effects underlying male homosexuality. But with the use of similar methods of family ascertainment, phenotyping, and genotyping, we were unable to confirm evidence for an Xq28-linked locus underlying male homosexuality."
A. Dean Byrd, PhD, Clinical Professor of Medicine at the University of Utah School of Medicine, wrote in his May 27, 2001 Salt Lake City Tribune article "The Innate-Immutable Argument Finds No Basis in Science":
"What is clear, however, is that the scientific attempts to demonstrate that homosexual attraction is biologically determined have failed. The major researchers now prominent in the scientific arena-themselves gay activists-have in fact arrived at such conclusions.
There is no support in the scientific research for the conclusion that homosexuality is biologically determined."