What you carry at this position in your DNA and what research says about it.
Breast Cancer Risk: 10/10 evidence, 58 publications.
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Sample genotype (not yours): A/A
Significantly Higher Breast Cancer Risk
Carrying two copies of the risk allele is linked to a roughly 1.6-fold increase in breast cancer risk, particularly for estrogen receptor-positive tumors.
What this genotype means: ~1.6x higher risk of breast cancer; Higher susceptibility to estrogen receptor-positive tumors; Altered TOX3 gene expression.
Sample genotype (not yours): A/G
Moderately Higher Breast Cancer Risk
Carrying one copy of the risk allele is associated with a moderate increase in breast cancer risk, primarily affecting estrogen receptor-positive tumors.
What this genotype means: ~1.2x higher risk of breast cancer; Slightly higher susceptibility to estrogen receptor-positive tumors.
Sample genotype (not yours): G/G
Typical Breast Cancer Risk
Carrying two copies of the common allele is associated with a typical baseline risk of breast cancer.
What this genotype means: Typical baseline risk of breast cancer; No elevated risk from this specific genetic variant.
Gene function and biological role
Gene Function: CASC16, also known as cancer susceptibility 16, is a non-coding RNA gene on chromosome 16. It is heavily implicated in breast cancer susceptibility, particularly estrogen receptor-positive subtypes [1]. Studies suggest it may regulate the expression of nearby genes like TOX3, which is involved in chromatin remodeling and cell survival [2]. Alterations in this genomic region can influence how mammary cells grow and respond to hormonal signals [3].
Variant Impact: Located near the CASC16 and TOX3 genes, rs3803662 is a non-coding genetic change. This variant does not change a protein directly but instead alters regulatory regions that control gene expression levels [4]. The risk-associated A allele is linked to higher expression of nearby genes, which can promote tumor development [2]. It is one of the most strongly and consistently replicated low-penetrance breast cancer risk variants identified in genome-wide association studies [5].
Key Insight: This variant is one of the most strongly replicated genetic risk factors for breast cancer, specifically affecting estrogen receptor-positive tumors.
Location: CASC16. Chromosome 16 · 16q12.1 · 52.6 Mb
Estimated prevalence by population
Global 14.9%; European 7.2%; African 26.8%; East Asian 38.4%; South Asian 7.0%; Americas 14.8%; Ashkenazi 15.1%; Finnish 10.7%; Middle Eastern 15.3%; Amish 9.4%.
Estimated AA genotype frequency.
Global 43.8%; European 39.2%; African 49.9%; East Asian 47.1%; South Asian 38.9%; Americas 47.3%; Ashkenazi 47.5%; Finnish 44.0%; Middle Eastern 47.6%; Amish 42.6%.
Estimated AG genotype frequency.
Global 41.2%; European 53.6%; African 23.2%; East Asian 14.5%; South Asian 54.1%; Americas 37.9%; Ashkenazi 37.4%; Finnish 45.3%; Middle Eastern 37.1%; Amish 48.0%.
Estimated GG genotype frequency.
Showing effects for A/A genotype
Cancer & Oncology
Breast Cancer
Breast Cancer. Evidence: 10/10. Source: PMID 17529967. ~1.6x Higher Risk.
Male Breast Cancer. Evidence: 10/10. Source: PMID 23001122. Increased Risk.
Showing effects for A/G genotype
Breast Cancer. Evidence: 10/10. Source: PMID 17529967. ~1.2x Higher Risk.
Male Breast Cancer. Evidence: 10/10. Source: PMID 23001122. Slightly Increased.
Showing effects for G/G genotype
Breast Cancer. Evidence: 10/10. Source: PMID 17529967. Typical Risk.
Male Breast Cancer. Evidence: 10/10. Source: PMID 23001122. Typical Risk.
What this genotype means for the people who carry it
Sample genotype: AA (Homozygous)
A significantly higher risk of developing breast cancer, particularly the estrogen receptor-positive subtype, is associated with the A/A genotype [1]. Large-scale genome-wide association studies have consistently identified this variant as a key low-penetrance risk factor [2]. Research indicates that carriers of this genotype may also face a younger age of diagnosis, often before age 60 [3]. The genetic change is thought to alter the expression of the nearby TOX3 gene, which plays a role in cell survival and growth [4]. While this variant increases susceptibility, it is considered a low-penetrance marker, meaning it is only one of many genetic and environmental factors that contribute to overall risk [5]. This means that many people with this genotype will never develop breast cancer. Understanding this risk can help inform personalized wellness and screening discussions with healthcare providers.
~1.6x higher risk of breast cancer; Higher susceptibility to estrogen receptor-positive tumors; Altered TOX3 gene expression.
Sample genotype: AG (Heterozygous)
A moderate increase in breast cancer risk is associated with the A/G genotype compared to the typical baseline [1]. This intermediate risk is estimated to be around 1.2-fold higher than baseline, driven by the single copy of the risk-associated A allele [2]. The association is strongest for estrogen receptor-positive breast cancers, which are the most common type of breast tumors [3]. Researchers believe this variant influences the activity of nearby genes involved in mammary cell development [4]. Because this is a common, low-penetrance variant, it does not guarantee that cancer will develop, but rather contributes to a person's overall polygenic risk profile [5]. Most individuals with this genotype do not go on to develop the condition. It serves as one piece of a larger puzzle that includes lifestyle, family history, and other genetic markers.
~1.2x higher risk of breast cancer; Slightly higher susceptibility to estrogen receptor-positive tumors.
Sample genotype: GG (Homozygous)
Typical baseline risk of breast cancer is expected for those with the G/G genotype [1]. This genotype is the most common in many populations and is considered the normal reference standard in genetic risk studies [2]. It does not carry the elevated risk for estrogen receptor-positive tumors seen in carriers of the A allele [3]. However, having a typical genetic risk at this specific locus does not mean zero risk, as lifestyle, environment, and other genetic factors still play a major role in overall breast health [4]. Knowing this baseline status provides a helpful starting point for understanding personal health.
Typical baseline risk of breast cancer; No elevated risk from this specific genetic variant.
Variants with shared genetic architecture
rs4784227 · CASC16 Gene · Chr 16 · Breast Cancer Risk
rs12443621 · TOX3 Gene · Chr 16 · Breast Cancer Risk
rs10941679 · FGF10 Gene · Chr 5 · Breast Cancer Risk
Relationships between SNPs and prognosis of breast cancer and pathogenic mechanism.
Genome-wide association study identifies novel breast cancer susceptibility loci.
A combined analysis of genome-wide association studies in breast cancer.
Combined effect of low-penetrant SNPs on breast cancer risk.
Content updated August 2026.
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