What you carry at this position in your DNA and what research says about it.
Inflammatory Disease Risk: 10/10 evidence, 24 publications.
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Sample genotype (not yours): C/C
Lower Risk of Inflammatory Bowel Disease and Asthma
Carrying two copies of the C allele is associated with a lower likelihood of developing Crohn's disease, ulcerative colitis, and asthma.
What this genotype means: Lower risk of Crohn's disease and ulcerative colitis; Reduced susceptibility to asthma and seasonal allergies.
Sample genotype (not yours): C/T
Typical Risk of Inflammatory Bowel Disease and Asthma
Carrying one copy of the T allele results in intermediate risk levels for inflammatory bowel and respiratory conditions.
What this genotype means: Average risk for inflammatory bowel conditions; Typical susceptibility to asthma and common allergies.
Sample genotype (not yours): T/T
Higher Risk of Inflammatory Bowel Disease and Asthma
Carrying two copies of the T allele is linked to an increased susceptibility to chronic gut inflammation and respiratory allergies.
What this genotype means: Increased risk of Crohn's disease and ulcerative colitis; Higher susceptibility to asthma and allergic rhinitis.
Gene function and biological role
Gene Function: The SMAD3 gene encodes a intracellular protein that transmits chemical signals from the cell surface to the nucleus [1]. It plays a central role in the transforming growth factor-beta signaling pathway, which regulates cell growth, tissue regeneration, and immune system responses [1]. Proper functioning of this pathway is crucial for maintaining the protective barrier of the gut and controlling inflammation [2]. Disruptions or variations in SMAD3 can lead to abnormal immune responses, contributing to chronic inflammatory conditions [2]. It is highly active in immune cells and tissues throughout the digestive and respiratory tracts.
Variant Impact: The rs17293632 variant is an intronic change located within an enhancer region of the SMAD3 gene [2]. This genetic change alters the binding efficiency of transcription factors, directly influencing how much SMAD3 protein the cell produces [2]. The T allele is associated with altered immune cell behavior and increased susceptibility to several chronic inflammatory disorders [3]. Conversely, the C allele maintains typical enhancer function and baseline gene expression [2]. This single-nucleotide variation serves as a key genetic link between respiratory allergies and gastrointestinal inflammation.
Key Insight: This genetic variant acts as a shared biological bridge between the gut and the lungs, showing how closely related our digestive and respiratory immune defenses are.
Location: SMAD3. Chromosome 15 · 15q22.33 · 67.2 Mb
Estimated prevalence by population
Global 69.1%; European 58.2%; African 87.4%; East Asian 94.6%; South Asian 73.2%; Americas 71.4%; Ashkenazi 62.5%; Finnish 53.9%; Middle Eastern 62.6%; Amish 48.2%.
Estimated CC genotype frequency.
Global 27.3%; European 36.2%; African 12.2%; East Asian 5.4%; South Asian 24.7%; Americas 26.2%; Ashkenazi 33.1%; Finnish 39.0%; Middle Eastern 33.1%; Amish 42.5%.
Estimated CT genotype frequency.
Global 3.6%; European 5.6%; African 0.4%; East Asian 0.1%; South Asian 2.1%; Americas 2.4%; Ashkenazi 4.4%; Finnish 7.1%; Middle Eastern 4.4%; Amish 9.4%.
Estimated TT genotype frequency.
Showing effects for C/C genotype
Immunity & Inflammation
Chronic Inflammatory Diseases
Crohn's Disease. Evidence: 7/10. Source: PMID 21102463. Lower Risk.
Ulcerative Colitis. Evidence: 7/10. Source: PMID 26192919. Lower Risk.
Allergy & Hypersensitivity
Allergic Rhinitis. Evidence: 8/10. Source: PMID 31361310. Lower Risk.
Organ Systems
Respiratory Health
Asthma. Evidence: 10/10. Source: PMID 29273806. Lower Risk.
Nasal Polyps. Evidence: 8/10. Source: PMID 41213931. Increased Risk.
Digestive & Gut Disorders
Hemorrhoids. Evidence: 6/10. Source: PMID 33888516. Lower Risk.
Showing effects for C/T genotype
Crohn's Disease. Evidence: 7/10. Source: PMID 21102463. Typical Risk.
Ulcerative Colitis. Evidence: 7/10. Source: PMID 26192919. Typical Risk.
Allergic Rhinitis. Evidence: 8/10. Source: PMID 31361310. Typical Risk.
Asthma. Evidence: 10/10. Source: PMID 29273806. Typical Risk.
Nasal Polyps. Evidence: 8/10. Source: PMID 41213931. Typical Risk.
Hemorrhoids. Evidence: 6/10. Source: PMID 33888516. Typical Risk.
Showing effects for T/T genotype
Crohn's Disease. Evidence: 7/10. Source: PMID 21102463. ~1.26x Higher Risk.
Ulcerative Colitis. Evidence: 7/10. Source: PMID 26192919. ~1.16x Higher Risk.
Allergic Rhinitis. Evidence: 8/10. Source: PMID 31361310. Increased Risk.
Asthma. Evidence: 10/10. Source: PMID 29273806. Increased Risk.
Nasal Polyps. Evidence: 8/10. Source: PMID 41213931. Lower Risk.
Hemorrhoids. Evidence: 6/10. Source: PMID 33888516. Increased Risk.
What this genotype means for the people who carry it
Sample genotype: CC (Homozygous)
This genotype is the most common globally and is associated with standard baseline levels of SMAD3 expression [1]. Large-scale genome-wide association studies show that individuals with this genotype have a lower risk of developing Crohn's disease and ulcerative colitis compared to those carrying the T allele [2]. The protective effect extends to the respiratory system, where it is linked to a reduced susceptibility to asthma and allergic rhinitis [3]. However, this genotype is associated with a slightly higher risk of developing chronic rhinosinusitis with nasal polyps [4]. Overall, the immune system maintains a balanced inflammatory profile in both the gut and the respiratory tract.
Lower risk of Crohn's disease and ulcerative colitis; Reduced susceptibility to asthma and seasonal allergies.
Sample genotype: CT (Heterozygous)
This heterozygous genotype represents a middle ground for immune-mediated inflammatory risks. Having one copy of the T allele slightly alters SMAD3 enhancer activity, leading to intermediate expression levels of the protein [1]. Research indicates that individuals with this genotype have typical risk levels for Crohn's disease, ulcerative colitis, and asthma, sitting between the lower-risk C/C genotype and the higher-risk T/T genotype [2]. Allergic responses, such as hay fever and elevated eosinophil counts, are generally near the population average [3]. This balanced genetic profile means that environmental factors and overall lifestyle play a more dominant role in determining actual health outcomes.
Average risk for inflammatory bowel conditions; Typical susceptibility to asthma and common allergies.
Sample genotype: TT (Homozygous)
This genotype is associated with altered SMAD3 enhancer function, which can impact how immune cells regulate inflammatory responses [1]. Multiple large-scale genetic studies have confirmed that carrying two copies of the T allele significantly increases the risk of developing Crohn's disease and ulcerative colitis [2]. The same genetic variation is also a well-documented risk factor for asthma and allergic rhinitis, likely due to its influence on systemic immune pathways [3]. Additionally, individuals with this genotype tend to have higher blood eosinophil counts, which are key white blood cells involved in allergic inflammation [4]. Conversely, this genotype offers a slight protective effect against nasal polyps.
Increased risk of Crohn's disease and ulcerative colitis; Higher susceptibility to asthma and allergic rhinitis.
Variants with shared genetic architecture
rs56062135 · SMAD3 Gene · Chr 15 · Heart Disease Risk
rs35874463 · SMAD3 Gene · Chr 15 · Heart and Body Structure
rs17228212 · SMAD3 Gene · Chr 15 · Heart Disease Risk
131 genetic loci highlight immunological pathways and tissues in nasal polyposis and asthma.
The Polygenic and Monogenic Basis of Blood Traits and Diseases.
Content updated August 2026.
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