What you carry at this position in your DNA and what research says about it.
Autoimmune Disease Risk: 9/10 evidence, 23 publications.
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Sample genotype (not yours): A/A
Higher Psoriasis and Rheumatoid Arthritis Risk
Carrying two copies of the common A allele increases the risk of psoriasis and rheumatoid arthritis. Conversely, this genotype provides natural protection against inflammatory bowel diseases.
What this genotype means: Elevated risk of psoriasis; Elevated risk of rheumatoid arthritis; Reduced risk of inflammatory bowel disease.
Sample genotype (not yours): A/C
Typical Autoimmune Risk Profile
Carrying one copy of each allele balances the genetic risks. This results in a typical, average risk profile for both skin and bowel inflammatory conditions.
What this genotype means: Average risk of psoriasis; Average risk of inflammatory bowel disease; Typical rheumatoid arthritis risk.
Sample genotype (not yours): C/C
Higher Inflammatory Bowel Disease Risk
Carrying two copies of the rare C allele increases the risk of Crohn's disease and ulcerative colitis. However, it provides strong protection against psoriasis and rheumatoid arthritis.
What this genotype means: Elevated risk of Crohn's disease; Elevated risk of ulcerative colitis; Reduced risk of psoriasis and rheumatoid arthritis.
Gene function and biological role
Gene Function: The TYK2 gene encodes tyrosine kinase 2, a member of the Janus kinase family that plays a critical role in immune system signaling [1]. It helps transmit chemical signals from cytokine receptors on the cell surface to the nucleus, activating downstream pathways essential for fighting infections [2]. However, overactive signaling through this pathway can lead to chronic inflammation and autoimmune disorders [1]. Because of its central role in regulating inflammatory cytokines like interleukins and interferons, TYK2 is a major therapeutic target for autoimmune diseases [2].
Variant Impact: The rs12720356 variant, also known as p.Ile684Ser, is a missense mutation in the regulatory pseudokinase domain of the TYK2 protein. This genetic change replaces the amino acid isoleucine with serine, which reduces the overall abundance of the TYK2 protein in immune cells [1]. While this alteration impairs certain signaling pathways, it does not cause severe immunodeficiency [3]. Instead, it creates a complex genetic trade-off, altering susceptibility to different types of autoimmune diseases depending on which allele is inherited [1].
Key Insight: This variant acts as a genetic seesaw, where protecting against skin and joint inflammation simultaneously increases the risk of inflammatory bowel diseases.
Location: TYK2. Chromosome 19 · 19p13.2 · 10.4 Mb
Estimated prevalence by population
Global 89.0%; European 83.6%; African 97.0%; East Asian 100.0%; South Asian 98.2%; Americas 89.6%; Ashkenazi 81.7%; Finnish 84.2%; Middle Eastern 85.6%; Amish 90.2%.
Estimated AA genotype frequency.
Global 10.6%; European 15.7%; African 3.0%; East Asian 0.0%; South Asian 1.8%; Americas 10.1%; Ashkenazi 17.4%; Finnish 15.1%; Middle Eastern 13.8%; Amish 9.6%.
Estimated AC genotype frequency.
Global 0.4%; European 0.7%; African < 0.1%; East Asian 0.0%; South Asian < 0.1%; Americas 0.3%; Ashkenazi 0.9%; Finnish 0.7%; Middle Eastern 0.6%; Amish 0.3%.
Estimated CC genotype frequency.
Showing effects for A/A genotype
Organ Systems
Skin & Hair Conditions
Psoriasis. Evidence: 8/10. Source: PMID 20953190. ~1.4x Higher Risk.
Immunity & Inflammation
Chronic Inflammatory Diseases
Crohn's Disease. Evidence: 9/10. Source: PMID 21102463. Lower Risk.
Ulcerative Colitis. Evidence: 9/10. Source: PMID 26192919. Lower Risk.
Autoimmune Diseases
Rheumatoid Arthritis. Evidence: 8/10. Source: PMID 33686288. ~1.15x Higher Risk.
Showing effects for A/C genotype
Psoriasis. Evidence: 8/10. Source: PMID 20953190. Typical Risk.
Crohn's Disease. Evidence: 9/10. Source: PMID 21102463. Typical Risk.
Ulcerative Colitis. Evidence: 9/10. Source: PMID 26192919. Typical Risk.
Rheumatoid Arthritis. Evidence: 8/10. Source: PMID 33686288. Typical Risk.
Showing effects for C/C genotype
Psoriasis. Evidence: 8/10. Source: PMID 20953190. Lower Risk.
Crohn's Disease. Evidence: 9/10. Source: PMID 21102463. ~1.15x Higher Risk.
Ulcerative Colitis. Evidence: 9/10. Source: PMID 26192919. ~1.17x Higher Risk.
Rheumatoid Arthritis. Evidence: 8/10. Source: PMID 33686288. Lower Risk.
What this genotype means for the people who carry it
Sample genotype: AA (Homozygous)
Inheriting two copies of the ancestral A allele maintains standard levels of TYK2 protein abundance and signaling. This active signaling state is associated with a significantly higher risk of developing psoriasis, with studies showing an odds ratio of approximately 1.4 [1]. It also moderately elevates the risk of rheumatoid arthritis [2]. On the positive side, this active immune state helps protect against gastrointestinal inflammation, resulting in a lower risk of Crohn's disease and ulcerative colitis [3]. This is the most common genotype globally, representing the baseline immune profile for the majority of the population.
Elevated risk of psoriasis; Elevated risk of rheumatoid arthritis; Reduced risk of inflammatory bowel disease.
Sample genotype: AC (Heterozygous)
Possessing one A allele and one C allele results in intermediate TYK2 protein levels and balanced immune signaling. This heterozygous state avoids the elevated risk of psoriasis and rheumatoid arthritis associated with the A/A genotype [1] [2]. At the same time, it does not carry the high risk of inflammatory bowel diseases seen in those with two copies of the C allele [3]. Because the opposing genetic risks effectively cancel each other out, overall susceptibility to these autoimmune conditions remains close to the population average. This balanced profile is found in about ten percent of people of European ancestry.
Average risk of psoriasis; Average risk of inflammatory bowel disease; Typical rheumatoid arthritis risk.
Sample genotype: CC (Homozygous)
Inheriting two copies of the variant C allele leads to reduced TYK2 protein abundance and altered immune signaling. This reduced activity significantly lowers the risk of developing psoriasis [1] and rheumatoid arthritis [2]. However, this altered immune state is less effective at maintaining gut homeostasis, leading to an increased risk of inflammatory bowel diseases [3]. Specifically, this genotype is linked to a higher risk of both Crohn's disease and ulcerative colitis [3]. This homozygous minor genotype is very rare, occurring in less than one percent of the global population.
Elevated risk of Crohn's disease; Elevated risk of ulcerative colitis; Reduced risk of psoriasis and rheumatoid arthritis.
Variants with shared genetic architecture
rs34536443 · TYK2 Gene · Chr 19 · Autoimmunity and Infection Risk
rs2304256 · TYK2 Gene · Chr 19 · Autoimmune Disease Risk
rs280519 · TYK2 Gene · Chr 19 · Autoimmune Disease Risk
Resolving TYK2 locus genotype-to-phenotype differences in autoimmunity.
Two rare disease-associated Tyk2 variants are catalytically impaired but signaling competent.
Content updated August 2026.
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