What you carry at this position in your DNA and what research says about it.
Fetal Hemoglobin: 9/10 evidence, 38 publications.
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Sample genotype (not yours): T/T
Lower Fetal Hemoglobin and Higher Cholesterol
Linked to lower levels of protective fetal hemoglobin and higher blood cholesterol. It is also associated with slightly higher blood sugar markers.
What this genotype means: Lower levels of protective fetal hemoglobin; Higher LDL and total cholesterol; Slightly higher blood sugar markers.
Sample genotype (not yours): C/C
Higher Fetal Hemoglobin and Lower Cholesterol
Associated with significantly higher levels of protective fetal hemoglobin and lower blood cholesterol. It is also linked to lower blood sugar and liver enzyme levels.
What this genotype means: Significantly higher protective fetal hemoglobin; Lower LDL and total cholesterol; Lower blood sugar and liver enzyme levels.
Sample genotype (not yours): T/C
Intermediate Fetal Hemoglobin and Cholesterol Levels
Carries one copy of the allele that increases protective fetal hemoglobin and lowers cholesterol. This results in intermediate levels of both markers.
What this genotype means: Intermediate levels of fetal hemoglobin; Typical cholesterol and blood sugar levels; Balanced blood cell counts.
Gene function and biological role
Gene Function: The HBS1L-MYB intergenic region on chromosome 6 is a critical regulatory hub that controls the development of red blood cells [1]. This region contains enhancer elements that interact over long genomic distances with the MYB gene, which encodes a transcription factor essential for blood cell production [2]. By modulating MYB expression, variants in this region influence the rate at which blood precursor cells mature [3]. This regulatory control directly impacts the production of different types of hemoglobin, particularly fetal hemoglobin [4]. Consequently, this locus is one of the most significant genetic modifiers of severity in blood disorders like sickle cell disease and beta-thalassemia [5].
Variant Impact: The rs9399137 variant is a single nucleotide polymorphism located within the HBS1L-MYB intergenic region [1]. This specific genetic change is in complete linkage disequilibrium with a functional 3-base pair deletion that alters transcription factor binding sites [3]. By disrupting these binding sites, the variant reduces the activity of a long-range enhancer that regulates the MYB gene [3]. This reduction in MYB expression slows down erythroid cell maturation, which allows more fetal hemoglobin to persist into adulthood [3]. Beyond hemoglobin, this variant has systemic pleiotropic effects on platelet counts, blood sugar regulation, and lipid metabolism [6].
Key Insight: This variant acts as a genetic volume dial for fetal hemoglobin, which normally shuts off after birth but can protect against severe blood disorders if kept active. It also simultaneously influences metabolic markers like cholesterol and blood sugar.
Location: MYB. Chromosome 6 · 6q23.3 · 135.1 Mb
Estimated prevalence by population
Global 65.4%; European 54.8%; African 86.9%; East Asian 55.5%; South Asian 79.4%; Americas 70.3%; Ashkenazi 58.7%; Finnish 42.6%; Middle Eastern 65.0%; Amish 70.5%.
Estimated TT genotype frequency.
Global 29.9%; European 38.4%; African 12.7%; East Asian 38.0%; South Asian 19.5%; Americas 27.1%; Ashkenazi 35.8%; Finnish 45.4%; Middle Eastern 31.3%; Amish 26.9%.
Estimated TC genotype frequency.
Global 4.7%; European 6.7%; African 0.5%; East Asian 6.5%; South Asian 1.2%; Americas 2.6%; Ashkenazi 5.5%; Finnish 12.1%; Middle Eastern 3.8%; Amish 2.6%.
Estimated CC genotype frequency.
Showing effects for T/T genotype
Rare Genetic Disorders
Hemoglobinopathies
Fetal Hemoglobin. Evidence: 9/10. Source: PMID 17592125. Lower.
Cardiovascular & Metabolic
Lipids & Cholesterol
LDL Cholesterol. Evidence: 8/10. Source: PMID 34887591. Higher.
Diabetes & Glycemic Traits
HbA1c. Evidence: 10/10. Source: PMID 24647736. Higher.
Blood Cell & Iron Traits
Platelet Count. Evidence: 10/10. Source: PMID 19853236. Lower.
Anemia. Evidence: 8/10. Source: PMID 39024449. Lower Risk.
Organ Systems
Liver Function & Disease
AST Liver Enzyme. Evidence: 9/10. Source: PMID 33339817. Higher.
Showing effects for C/C genotype
Fetal Hemoglobin. Evidence: 9/10. Source: PMID 17592125. Higher.
LDL Cholesterol. Evidence: 8/10. Source: PMID 34887591. Lower.
HbA1c. Evidence: 10/10. Source: PMID 24647736. Lower.
Platelet Count. Evidence: 10/10. Source: PMID 19853236. Higher.
Anemia. Evidence: 8/10. Source: PMID 39024449. Slightly Higher Risk.
AST Liver Enzyme. Evidence: 9/10. Source: PMID 33339817. Lower.
Showing effects for T/C genotype
Fetal Hemoglobin. Evidence: 9/10. Source: PMID 17592125. Intermediate.
LDL Cholesterol. Evidence: 8/10. Source: PMID 34887591. Intermediate.
HbA1c. Evidence: 10/10. Source: PMID 24647736. Intermediate.
Platelet Count. Evidence: 10/10. Source: PMID 19853236. Intermediate.
Anemia. Evidence: 8/10. Source: PMID 39024449. Typical Risk.
AST Liver Enzyme. Evidence: 9/10. Source: PMID 33339817. Intermediate.
What this genotype means for the people who carry it
Sample genotype: TT (Homozygous)
This genotype is associated with the lowest levels of protective fetal hemoglobin in adults [1]. Because fetal hemoglobin levels remain low, individuals with blood disorders like sickle cell disease or beta-thalassemia may experience more severe symptoms [2]. In addition to blood traits, this genotype is linked to higher levels of total and LDL cholesterol [3]. It also correlates with slightly elevated HbA1c, a marker of long-term blood sugar levels [4]. Liver enzyme levels, specifically aspartate aminotransferase, also tend to be higher [5]. Conversely, this profile is associated with a lower risk of general anemia [3]. Overall, this genotype represents the common baseline profile with slightly higher metabolic risks.
Lower levels of protective fetal hemoglobin; Higher LDL and total cholesterol; Slightly higher blood sugar markers.
Sample genotype: CC (Homozygous)
This genotype is strongly associated with the highest levels of protective fetal hemoglobin [1]. In individuals with sickle cell disease or beta-thalassemia, this high fetal hemoglobin level is known to significantly reduce disease severity and pain crises [2]. On a metabolic level, this genotype is linked to lower total and LDL cholesterol, reducing the risk of high cholesterol [3]. It is also associated with lower HbA1c levels, indicating better long-term blood sugar regulation [4]. Furthermore, liver enzyme levels such as aspartate aminotransferase tend to be lower [5]. However, this genotype is also associated with higher platelet counts and a slightly increased risk of general anemia [3]. Despite the slight anemia risk, the combination of high fetal hemoglobin and favorable lipid and blood sugar profiles makes this genotype highly advantageous.
Significantly higher protective fetal hemoglobin; Lower LDL and total cholesterol; Lower blood sugar and liver enzyme levels.
Sample genotype: TC (Heterozygous)
This heterozygous genotype represents an intermediate genetic profile for both blood and metabolic traits [1]. Individuals carrying one copy of the C allele show moderate increases in protective fetal hemoglobin compared to the common T/T baseline [2]. This intermediate level can provide a partial buffering effect against the severity of certain hemoglobin disorders [3]. On the metabolic side, this genotype is associated with intermediate levels of LDL and total cholesterol [4]. It also results in moderate levels of HbA1c and liver enzymes like aspartate aminotransferase [5]. Platelet counts and red blood cell distribution width also fall into a typical, intermediate range [4]. This balanced profile avoids the higher metabolic risks of the T/T genotype while carrying some of the protective benefits of the C allele.
Intermediate levels of fetal hemoglobin; Typical cholesterol and blood sugar levels; Balanced blood cell counts.
Variants with shared genetic architecture
rs9402686 · HBS1L Gene · Chr 6 · Fetal Hemoglobin Levels
rs4895441 · HBS1L Gene · Chr 6 · Fetal Hemoglobin Levels
rs11759553 · HBS1L Gene · Chr 6 · Fetal Hemoglobin Levels
Diversity and scale: Genetic architecture of 2068 traits in the VA Million Veteran Program.
A cross-population atlas of genetic associations for 220 human phenotypes.
Content updated August 2026.
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