What you carry at this position in your DNA and what research says about it.
Breast Cancer Risk: 8/10 evidence, 17 publications.
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Sample genotype (not yours): C/C
Higher Breast Cancer Risk
Two copies of the C allele are associated with a slightly elevated risk of developing breast cancer. This variant is very common in European populations but has little impact on other ancestries.
What this genotype means: Slightly higher risk of breast cancer; Slightly higher risk of ER-negative breast cancer; Slightly shorter average height.
Sample genotype (not yours): C/T
Slightly Elevated Breast Cancer Risk
One copy of each allele is associated with a minor increase in breast cancer risk compared to the protective genotype. This intermediate risk is common among individuals of European descent.
What this genotype means: Slightly elevated risk of breast cancer; Slightly elevated risk of ER-negative breast cancer.
Sample genotype (not yours): T/T
Typical Breast Cancer Risk
Two copies of the T allele are associated with the lowest risk of breast cancer at this genetic locus. This genotype provides a protective effect compared to the more common C/C genotype.
What this genotype means: Lowest genetic risk of breast cancer at this locus; Slightly taller average height.
Gene function and biological role
Gene Function: RAD51B encodes a protein that is a member of the RAD51 protein family, which is essential for DNA repair [1]. Specifically, this protein plays a crucial role in homologous recombination, a mechanism cells use to repair double-strand breaks in DNA [1]. By forming complexes with other RAD51 paralogs, it helps maintain genomic stability and prevents harmful mutations from accumulating [2]. Disruptions or variations in this gene can impair the cell's ability to fix DNA damage, which can lead to uncontrolled cell division and tumor development [2]. Consequently, RAD51B has been widely studied as a key susceptibility gene for various cancers, particularly breast cancer [3].
Variant Impact: The rs999737 variant is located within an intron of the RAD51B gene on chromosome 14. This specific genetic change does not alter the protein sequence directly but is thought to influence how the gene is expressed or spliced. Large-scale genetic studies have consistently linked this variant to breast cancer susceptibility in populations of European descent [4]. The common C allele is associated with a slightly higher risk of developing breast cancer, including estrogen-receptor-negative subtypes [5]. Conversely, the less common T allele appears to have a protective effect, reducing the likelihood of tumor development [6].
Key Insight: Although this variant is a well-established risk factor for breast cancer in people of European ancestry, it is extremely rare or virtually absent in East Asian and African populations, meaning it does not play a significant role in their risk profiles.
Location: RAD51B. Chromosome 14 · 14q24.1 · 68.6 Mb
Estimated prevalence by population
Global 71.6%; European 58.3%; African 91.4%; East Asian 99.4%; South Asian 76.0%; Americas 69.6%; Ashkenazi 76.5%; Finnish 65.2%; Middle Eastern 85.6%; Amish 55.8%.
Estimated CC genotype frequency.
Global 25.2%; European 36.1%; African 8.4%; East Asian 0.6%; South Asian 22.4%; Americas 27.6%; Ashkenazi 22.0%; Finnish 31.1%; Middle Eastern 13.8%; Amish 37.8%.
Estimated CT genotype frequency.
Global 3.3%; European 5.6%; African 0.2%; East Asian < 0.1%; South Asian 1.6%; Americas 2.7%; Ashkenazi 1.6%; Finnish 3.7%; Middle Eastern 0.6%; Amish 6.4%.
Estimated TT genotype frequency.
Showing effects for C/C genotype
Cancer & Oncology
Breast Cancer
Breast Cancer. Evidence: 8/10. Source: PMID 19330030. ~1.1x Increased Risk.
ER-Negative Breast Cancer. Evidence: 8/10. Source: PMID 29058716. ~1.09x Increased Risk.
Lifestyle, Fitness & Aging
Height & Body Size
Height. Evidence: 6/10. Source: PMID 36224396. Slightly Shorter.
Showing effects for C/T genotype
Breast Cancer. Evidence: 8/10. Source: PMID 19330030. Slightly Increased.
ER-Negative Breast Cancer. Evidence: 8/10. Source: PMID 29058716. Slightly Increased.
Height. Evidence: 6/10. Source: PMID 36224396. Typical.
Showing effects for T/T genotype
Breast Cancer. Evidence: 8/10. Source: PMID 19330030. Typical Risk.
ER-Negative Breast Cancer. Evidence: 8/10. Source: PMID 29058716. Typical Risk.
Height. Evidence: 6/10. Source: PMID 36224396. Slightly Taller.
What this genotype means for the people who carry it
Sample genotype: CC (Homozygous)
Double copies of the C allele represent the genetic variant most strongly linked to breast cancer susceptibility at this locus [1]. Large-scale genome-wide association studies have shown that this genotype increases the relative risk of breast cancer by approximately 10% to 20% compared to those with the protective T/T genotype [2]. This elevated risk applies to both overall breast cancer and specific subtypes, such as estrogen-receptor-negative breast cancer [3]. The RAD51B gene is vital for repairing damaged DNA, and carrying the C/C genotype may slightly reduce the efficiency of this repair process over time [1]. However, because this is a common variant with a small individual effect size, it is not a direct cause of cancer on its own but rather one of many small genetic factors that contribute to overall risk [4]. In non-European populations, such as East Asian and African ancestries, this specific variant is extremely rare, so its associated risk is less relevant [5].
Slightly higher risk of breast cancer; Slightly higher risk of ER-negative breast cancer; Slightly shorter average height.
Sample genotype: CT (Heterozygous)
Inheriting one copy of each allele places individuals in an intermediate risk category for breast cancer [1]. Studies indicate that having a single C allele increases the risk of breast cancer slightly, though less than having two copies of the C allele [2]. This association has been replicated across multiple large cohorts, particularly in women of European ancestry [3]. The RAD51B gene plays a key role in homologous recombination, which repairs double-strand DNA breaks to prevent cellular mutations [1]. A single copy of the protective T allele may offer some mitigation, but the presence of the C allele still slightly elevates susceptibility [4]. This variant is also associated with a minor increase in the risk of estrogen-receptor-negative breast cancer [5]. Because the effect size of this variant is small, it should be viewed as a single piece of a much larger genetic and lifestyle puzzle [3].
Slightly elevated risk of breast cancer; Slightly elevated risk of ER-negative breast cancer.
Sample genotype: TT (Homozygous)
Two copies of the T allele are associated with a reduced risk of breast cancer compared to carrying the C allele [1]. Genome-wide association studies have consistently shown that the T/T genotype is protective, lowering the relative odds of developing breast cancer by about 10% to 20% compared to the C/C genotype [2]. This protective effect extends to various subtypes of the disease, including estrogen-receptor-negative breast cancer [3]. The RAD51B gene is crucial for maintaining genomic stability by repairing double-strand DNA breaks [4]. Individuals with the T/T genotype may benefit from highly efficient DNA repair processes mediated by this pathway [4]. While this genotype is favorable, it does not eliminate the overall risk of breast cancer, which is heavily influenced by other genetic variants, family history, and lifestyle factors [5].
Lowest genetic risk of breast cancer at this locus; Slightly taller average height.
Variants with shared genetic architecture
rs1950897 · RAD51B Gene · Chr 14 · Rheumatoid Arthritis Risk
rs2588809 · RAD51B Gene · Chr 14 · Breast Cancer Risk
rs911263 · RAD51B Gene · Chr 14 · Biliary Cholangitis Risk
Large-scale genotyping identifies 41 new loci associated with breast cancer risk.
Identification of ten variants associated with risk of estrogen-receptor-negative breast cancer.
Association analysis identifies 65 new breast cancer risk loci.
Genetic variants at 14q24.1 and breast cancer susceptibility: a fine-mapping study in Chinese women.
Content updated August 2026.
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