Sex-determining region Y protein

Function

During gestation, the cells of the primordial gonad that lie along the urogenital ridge are in a bipotential state, meaning they possess the ability to become either male cells (Sertoli and Leydig cells) or female cells (follicle cells and theca cells). SRY initiates testis differentiation by activating male-specific transcription factors that allow these bipotential cells to differentiate and proliferate. SRY accomplishes this by upregulating SOX9, a transcription factor with a DNA-binding site very similar to SRY's. SOX9 leads to the upregulation of fibroblast growth factor 9 (Fgf9), which in turn leads to further upregulation of SOX9. Once proper SOX9 levels are reached, the bipotential cells of the gonad begin to differentiate into Sertoli cells. Additionally, cells expressing SRY will continue to proliferate to form the primordial testis. This brief review constitutes the basic series of events, but there are many more factors that influence sex differentiation.

Action in the nucleus

The SRY protein consists of three main regions. The central region encompasses the high-mobility group (HMG) domain, which contains nuclear localization sequences and acts as the DNA-binding domain. The C-terminal domain has no conserved structure, and the N-terminal domain can be phosphorylated to enhance DNA-binding. The process begins with nuclear localization of SRY by acetylation of the nuclear localization signal regions, which allows for the binding of importin β and calmodulin to SRY, facilitating its import into the nucleus. Once in the nucleus, SRY and SF1 (steroidogenic factor 1, another transcriptional regulator) complex and bind to TESCO (testis-specific enhancer of Sox9 core), the testes-specific enhancer element of the Sox9 gene in Sertoli cell precursors, located upstream of the Sox9 gene transcription start site. Specifically, it is the HMG region of SRY that binds to the minor groove of the DNA target sequence, causing the DNA to bend and unwind. The establishment of this particular DNA "architecture" facilitates the transcription of the Sox9 gene. In the nucleus of Sertoli cells, SOX9 directly targets the Amh gene as well as the prostaglandin D2 synthase (Ptgds) gene. SOX9 binding to the enhancer near the Amh promoter allows for the synthesis of Amh while SOX9 binding to the Ptgds gene allows for the production of prostaglandin D2 (PGD₂). The reentry of SOX9 into the nucleus is facilitated by autocrine or paracrine signaling conducted by PGD₂. SOX9 protein then initiates a positive feedback loop, involving SOX9 acting as its own transcription factor and resulting in the synthesis of large amounts of SOX9.

SOX9 and testes differentiation

The SF-1 protein, on its own, leads to minimal transcription of the SOX9 gene in both the XX and XY bipotential gonadal cells along the urogenital ridge. However, binding of the SRY-SF1 complex to the testis-specific enhancer (TESCO) on SOX9 leads to significant up-regulation of the gene in only the XY gonad, while transcription in the XX gonad remains negligible. Part of this up-regulation is accomplished by SOX9 itself through a positive feedback loop; like SRY, SOX9 complexes with SF1 and binds to the TESCO enhancer, leading to further expression of SOX9 in the XY gonad. Two other proteins, FGF9 (fibroblast growth factor 9) and PDG2 (prostaglandin D2), also maintain this up-regulation. Although their exact pathways are not fully understood, they have been proven to be essential for the continued expression of SOX9 at the levels necessary for testes development.

SOX9 and SRY are believed to be responsible for the cell-autonomous differentiation of supporting cell precursors in the gonads into Sertoli cells, the beginning of testes development. These initial Sertoli cells, in the center of the gonad, are hypothesized to be the starting point for a wave of FGF9 that spreads throughout the developing XY gonad, leading to further differentiation of Sertoli cells via the up-regulation of SOX9. SOX9 and SRY are also believed to be responsible for many of the later processes of testis development (such as Leydig cell differentiation, sex cord formation, and formation of testis-specific vasculature), although exact mechanisms remain unclear. It has been shown, however, that SOX9, in the presence of PDG2, acts directly on Amh (encoding anti-Müllerian hormone) and is capable of inducing testis formation in XX mice gonads, indicating it is vital to testes development.

SRY disorders' influence on sex expression

Embryos are gonadally identical, regardless of genetic sex, until a certain point in development when the testis-determining factor causes male sex organs to develop. A typical male karyotype is XY, whereas a female's is XX. There are exceptions, however, in which SRY plays a major role. Individuals with Klinefelter syndrome inherit a normal Y chromosome and multiple X chromosomes, giving them a karyotype of XXY. Atypical genetic recombination during crossover, when a sperm cell is developing, can result in karyotypes that are not typical for their phenotypic expression.

Most of the time, when a developing sperm cell undergoes crossover during meiosis, the SRY gene stays on the Y chromosome. If the SRY gene is transferred to the X chromosome instead of staying on the Y chromosome, testis development will no longer occur. This is known as Swyer syndrome, characterized by an XY karyotype and a female phenotype. Individuals who have this syndrome have normally formed uteri and fallopian tubes, but the gonads are not functional. Swyer syndrome individuals are usually considered as females. On the other spectrum, XX male syndrome, also known as de la Chapelle syndrome, occurs when a body has 46:XX Karyotype and SRY attaches to one of them through translocation. People with XX male syndrome have a XX Karyotype but are male. Individuals with either of these syndromes can experience delayed puberty, infertility, and growth features of the opposite sex they identify with. XX male syndrome expressers may develop breasts, and those with Swyer syndrome may have facial hair.

While the presence or absence of SRY has generally determined whether or not testis development occurs, it has been suggested that there are other factors that affect the functionality of SRY. Therefore, there are individuals who have the SRY gene, but still develop as females, either because the gene itself is defective or mutated, or because one of the contributing factors is defective. This can happen in individuals exhibiting a XY, XXY, or XX SRY-positive karyotype

Additionally, other sex determining systems that rely on SRY beyond XY are the processes that come after SRY is present or absent in the development of an embryo. In a normal system, if SRY is present for XY, SRY will activate the medulla to develop gonads into testes. Testosterone will then be produced and initiate the development of other male sexual characteristics. Comparably, if SRY is not present for XX, there will be a lack of the SRY based on no Y chromosome. The lack of SRY will allow the cortex of embryonic gonads to develop into ovaries, which will then produce estrogen, and lead to the development of other female sexual characteristics.

Role in other diseases

SRY has been shown to interact with the androgen receptor and individuals with XY karyotype and a functional SRY gene can have an outwardly female phenotype due to an underlying androgen insensitivity syndrome (AIS). Individuals with AIS are unable to respond to androgens properly due to a defect in their androgen receptor gene, and affected individuals can have complete or partial AIS. SRY has also been linked to the fact that males are more likely than females to develop dopamine-related diseases such as schizophrenia and Parkinson's disease. SRY encodes a protein that controls the concentration of dopamine, the neurotransmitter that carries signals from the brain that control movement and coordination. Research in mice has shown that a mutation in SOX10, an SRY encoded transcription factor, is linked to the condition of Dominant megacolon in mice. This mouse model is being used to investigate the link between SRY and Hirschsprung disease, or congenital megacolon in humans. There is also a link between SRY encoded transcription factor SOX9 and campomelic dysplasia (CD). This missense mutation causes defective chondrogenesis, or the process of cartilage formation, and manifests as skeletal CD. Two thirds of 46,XY individuals diagnosed with CD have fluctuating amounts of male-to-female sex reversal.

Use in Olympic screening

One of the most controversial uses of this discovery was as a means for sex verification at the Olympic Games, under a system implemented by the International Olympic Committee in 1992. Athletes with an SRY gene were not permitted to participate as females, although all athletes in whom this was "detected" at the 1996 Summer Olympics were ruled false positives and were not disqualified. Specifically, eight female participants (out of a total of 3387) at these games were found to have the SRY gene. However, after further investigation of their genetic conditions, all these athletes were verified as female and allowed to compete. These athletes were found to have either partial or full androgen insensitivity, despite having an SRY gene, making them externally phenotypically female. In the late 1990s, a number of relevant professional societies in United States called for elimination of gender verification, including the American Medical Association, stating that the method used was uncertain and ineffective. Chromosomal screening was eliminated as of the 2000 Summer Olympics, but this was later followed by other forms of testing based on hormone levels. In March 2025 World Athletics announced it will do cheek swabbing tests for gender eligibility, specifically looking for the SRY gene, but that this would only be a first screen in determining eligibility, so that individuals with CAIS or Swyer's syndrome would not automatically be excluded from female competition.

On March 26, 2026, the IOC reinstated the use of one‑time SRY gene testing to determine eligibility for women's events, using saliva, cheek swab or blood samples.

Ongoing research

Despite the progress made during the past several decades in the study of sex determination, the SRY gene, and its protein, work is still being conducted to further understanding in these areas. There remain factors that need to be identified in the sex-determining molecular network, and the chromosomal changes involved in many other human sex-reversal cases are still unknown. Scientists continue to search for additional sex-determining genes, using techniques such as microarray screening of the genital ridge genes at varying developmental stages, mutagenesis screens in mice for sex-reversal phenotypes, and identifying the genes that transcription factors act on using chromatin immunoprecipitation.

Fetal development-knockout models

One of the knockout models for the SRY gene was done in pigs. Through the use of CRISPR technology the SRY gene was knocked out in male pigs. The target for the CRISPR technology is the high mobility group located on the SRY gene, causing a frameshift mutation in the SRY protein sequence. The research showed that with the knockout of SRY, both the internal and external genitalia were reversed. When the piglets were born they were genetically male but expressed female genitalia. However, the piglet reproductive system did not achieve maturation or become sexually competent. Another study done on mice used TALEN technology to produce an SRY knockout model. These mice expressed external and internal genitalia as well as a normal female level of circulating testosterone. These mice, despite having XY chromosomes, expressed a normal estrus cycle albeit with reduced fertility. Both of these studies highlighted the role that SRY plays in the development of the testes and other male reproductive organs.

SRY knock-in

CRISPR-Cas9 technology has been used to insert the SRY gene into XX individuals, thus creating a genetically female organism that is phenotypically male. Only a fragment of 14-kilobases of genomic DNA is necessary for the induction of testis. This alteration in addition to gene drives would allow for the induction of sterility to aid in population control of either unfavorable or invasive species. However, to utilize this knock-in, the relocation of the SRY gene onto the 17th chromosome (autosome) would be most efficient. These transgenic species would then be released into the wild to mate with the natural population, resulting in the creation of predominantly male offspring, thus decreasing reproductive rates. An autosomal SRY knock-in would result in a 75% SRY inheritance rate, whereas a 90% inheritance can be achieved when inserted into the t-complex on the 17th chromosome. Although, previously unsuccessful in mammals, more recent research has found that although thought to only contain single exon for the last 30 years, a second SRY exon has been located named SRY-T .