What you carry at this position in your DNA and what research says about it.
Autoimmune Disease Risk: 8/10 evidence, 28 publications.
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Sample genotype (not yours): C/C
Typical Autoimmune Disease Risk
Absence of the risk-increasing variant at this locus is associated with typical, baseline susceptibility to inflammatory disorders. Normal levels of B-cell signaling are maintained.
What this genotype means: Typical baseline risk for lupus and rheumatoid arthritis; Normal BLK gene expression levels.
Sample genotype (not yours): C/T
Slightly Increased Autoimmune Disease Risk
Presence of one copy of the T allele is associated with a slightly higher susceptibility to autoimmune conditions. This includes systemic lupus erythematosus and rheumatoid arthritis.
What this genotype means: Slightly elevated risk of systemic lupus erythematosus; Slightly elevated risk of rheumatoid arthritis.
Sample genotype (not yours): T/T
Increased Autoimmune Disease Risk
Presence of two copies of the T allele is associated with an increased susceptibility to autoimmune conditions. This includes systemic lupus erythematosus and rheumatoid arthritis.
What this genotype means: Elevated risk of systemic lupus erythematosus; Elevated risk of rheumatoid arthritis; Reduced BLK gene expression levels.
Gene function and biological role
Gene Function: The BLK gene encodes B lymphoid tyrosine kinase, a member of the Src family of tyrosine kinases [1]. This protein is primarily expressed in B cells, where it plays a critical role in B-cell receptor signaling, development, and tolerance [1] [2]. Proper functioning of BLK is essential for maintaining immune system balance and preventing the body from attacking its own tissues [2]. Reduced expression or activity of this kinase has been strongly implicated in the pathogenesis of various autoimmune diseases [3]. Genetic variations in the BLK region are among the most well-replicated risk factors for systemic lupus erythematosus and rheumatoid arthritis [3] [4].
Variant Impact: The rs2736340 variant is an intergenic single nucleotide polymorphism located near the BLK and FAM167A genes on chromosome 8 [3]. This variant is situated in a regulatory region where the T allele is associated with reduced expression of the BLK gene [1]. Lower BLK expression alters B-cell signaling thresholds, which can lead to a failure in immune tolerance and the survival of self-reactive B cells [1] [2]. Consequently, the T allele is widely recognized as a shared genetic risk factor for multiple autoimmune conditions, including systemic lupus erythematosus and rheumatoid arthritis [3] [4]. The frequency of this variant varies significantly across different ancestries, being particularly common in East Asian populations [5].
Key Insight: A single genetic variant near the BLK gene acts as a shared risk factor across multiple distinct autoimmune diseases, demonstrating a common biological pathway for conditions like lupus and rheumatoid arthritis.
Location: BLK. Chromosome 8 · 8p23.1 · 11.5 Mb
Origin Story: Natural selection studies suggest that some autoimmune risk alleles, including variants in the BLK region, may have been historically maintained in certain populations because they helped enhance immune responses against infectious pathogens.
Estimated prevalence by population
Global 56.4%; European 55.5%; African 74.3%; East Asian 8.1%; South Asian 42.3%; Americas 30.9%; Ashkenazi 73.3%; Finnish 52.7%; Middle Eastern 66.1%; Amish 65.1%.
Estimated CC genotype frequency.
Global 35.3%; European 38.0%; African 23.8%; East Asian 40.7%; South Asian 45.5%; Americas 49.4%; Ashkenazi 24.6%; Finnish 39.8%; Middle Eastern 30.4%; Amish 31.2%.
Estimated CT genotype frequency.
Global 8.3%; European 6.5%; African 1.9%; East Asian 51.2%; South Asian 12.2%; Americas 19.7%; Ashkenazi 2.1%; Finnish 7.5%; Middle Eastern 3.5%; Amish 3.7%.
Estimated TT genotype frequency.
Showing effects for C/C genotype
Immunity & Inflammation
Autoimmune Diseases
Lupus. Evidence: 8/10. Source: PMID 19838195. Typical Risk.
Rheumatoid Arthritis. Evidence: 8/10. Source: PMID 19503088. Typical Risk.
Systemic Sclerosis. Evidence: 8/10. Source: PMID 23740937. Typical Risk.
Chronic Inflammatory Diseases
Kawasaki Disease. Evidence: 6/10. Source: PMID 22446961. Typical Risk.
Organ Systems
Bone, Joint & Connective Tissue Health
Bone Mineral Density. Evidence: 6/10. Source: PMID 30048462. Typical.
Showing effects for C/T genotype
Lupus. Evidence: 8/10. Source: PMID 19838195. ~1.3x Increased Risk.
Rheumatoid Arthritis. Evidence: 8/10. Source: PMID 19503088. Increased Risk.
Systemic Sclerosis. Evidence: 8/10. Source: PMID 23740937. Increased Risk.
Kawasaki Disease. Evidence: 6/10. Source: PMID 22446961. Increased Risk.
Bone Mineral Density. Evidence: 6/10. Source: PMID 30048462. Associated.
Showing effects for T/T genotype
Lupus. Evidence: 8/10. Source: PMID 19838195. Increased Risk.
What this genotype means for the people who carry it
Sample genotype: CC (Homozygous)
Two copies of the C allele represent the baseline genetic profile for autoimmune susceptibility at this locus [1]. This genotype is not associated with the increased risk of systemic lupus erythematosus or rheumatoid arthritis seen in carriers of the T allele [1] [2]. Large-scale genetic studies have consistently demonstrated that individuals with the C/C genotype maintain typical levels of B lymphoid tyrosine kinase (BLK) expression, which is crucial for normal B-cell development and immune regulation [3]. By avoiding the expression-lowering T allele, this genotype helps maintain proper immune tolerance [3]. Consequently, individuals with this profile do not have the elevated risk of systemic sclerosis or Kawasaki disease linked to this variant [4] [5]. It is also associated with typical bone mineral density [6].
Typical baseline risk for lupus and rheumatoid arthritis; Normal BLK gene expression levels.
Sample genotype: CT (Heterozygous)
One copy of the T allele is linked to a moderate increase in the risk of developing several autoimmune disorders [1]. This genetic variant is located in the promoter region of the BLK gene, where it is thought to reduce the expression of B lymphoid tyrosine kinase [2]. Lower levels of this kinase can impair B-cell tolerance, making the immune system more likely to mistakenly target the body's own tissues [2]. Research has shown that carriers of this variant have an increased risk of systemic lupus erythematosus, with an odds ratio of approximately 1.33 [3]. It is also associated with a higher susceptibility to rheumatoid arthritis and systemic sclerosis [1] [4]. Additionally, some studies have linked this variant to Kawasaki disease, an inflammatory condition affecting blood vessels [5]. It may also influence markers of bone health, such as heel bone mineral density [6].
Slightly elevated risk of systemic lupus erythematosus; Slightly elevated risk of rheumatoid arthritis.
Sample genotype: TT (Homozygous)
Two copies of the T allele are associated with the highest genetic risk at this locus for several autoimmune conditions [1]. This genotype is linked to reduced expression of the BLK gene, which plays a vital role in B-cell signaling and immune tolerance [2]. When BLK expression is suppressed, the risk of developing systemic lupus erythematosus is significantly elevated [3]. Meta-analyses have confirmed that the T allele is a robust risk factor for rheumatoid arthritis and systemic sclerosis across multiple populations [1] [4]. Furthermore, this genotype is associated with a higher likelihood of developing Kawasaki disease, particularly in Asian populations [5]. It is also linked to variations in heel bone mineral density, reflecting the broad systemic effects of this locus [6].
Elevated risk of systemic lupus erythematosus; Elevated risk of rheumatoid arthritis; Reduced BLK gene expression levels.
Variants with shared genetic architecture
rs13277113 · BLK Gene · Chr 8 · Autoimmune Disease Risk
rs2254546 · BLK Gene · Chr 8 · Kawasaki Disease Risk
rs2476601 · PTPN22 Gene · Chr 1 · Autoimmune Disease Risk
Content updated August 2026.
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