What you carry at this position in your DNA and what research says about it.
Hartnup Disease and Amino Acids: 7/10 evidence, 13 publications.
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Sample genotype (not yours): G/G
Typical Amino Acid Transport and Low Hartnup Risk
Two copies of the common G allele indicate fully functional amino acid transporters. There is no elevated risk of Hartnup disease or related urinary amino acid imbalances.
What this genotype means: Fully functional B0AT1 transporter protein; Typical urinary amino acid excretion; No genetic predisposition to Hartnup disease.
Sample genotype (not yours): A/A
High Risk of Hartnup Disease and Aminoaciduria
Two copies of the A allele significantly impair amino acid absorption, leading to Hartnup disease and elevated amino acids in the urine. Symptoms can include skin rashes and sensitivity to light, especially during times of poor nutrition.
What this genotype means: High risk of developing Hartnup disease; Marked loss of neutral amino acids in urine; Increased susceptibility to niacin deficiency under stress.
Sample genotype (not yours): G/A
Hartnup Disease Carrier and Elevated Urinary Glycine
One copy of the A allele makes you a carrier of Hartnup disease, which is generally harmless. However, this genotype is associated with hyperglycinuria, causing elevated glycine levels in the urine.
What this genotype means: Carrier of recessive Hartnup disease; Predisposition to dominant hyperglycinuria; Slightly altered kidney biomarker levels.
Gene function and biological role
Gene Function: The SLC6A19 gene encodes a sodium-dependent neutral amino acid transporter known as B0AT1 [1]. This transporter is primarily expressed in the kidneys and the small intestine, where it plays a critical role in reabsorbing essential amino acids [1]. Specifically, it is responsible for transporting neutral amino acids like tryptophan, which the body uses to produce niacin (vitamin B3) [1]. When this gene functions poorly, amino acids are lost in the urine rather than being absorbed into the bloodstream [1]. This transport defect can lead to systemic nutrient deficiencies, particularly under conditions of poor diet or physical stress [1].
Variant Impact: The rs121434346 variant represents a missense mutation in the SLC6A19 gene, changing the amino acid aspartic acid to asparagine at position 173 (p.Asp173Asn) [1]. This specific change impairs the trafficking and surface expression of the B0AT1 transporter in epithelial cells [2]. As a result, the transport of neutral amino acids in both the renal tubules and intestinal mucosa is significantly reduced [1]. This variant is the most common mutation associated with Hartnup disorder in populations of European ancestry [3]. While many carriers remain asymptomatic, having two copies of this variant leads to a marked increase in urinary amino acid excretion [1].
Key Insight: Most people with the genetic signature for Hartnup disease never develop severe symptoms, as a standard modern diet typically provides enough pre-formed vitamins to bypass the absorption defect.
Location: SLC6A19. Chromosome 5 · 5p15.33 · 1.2 Mb
Estimated prevalence by population
Global 99.7%; European 99.4%; African 99.9%; East Asian 100.0%; South Asian 99.9%; Americas > 99.9%; Ashkenazi 99.8%; Finnish 100.0%; Middle Eastern 100.0%; Amish 100.0%.
Estimated GG genotype frequency.
Global 0.3%; European 0.6%; African 0.1%; East Asian 0.0%; South Asian 0.1%; Americas < 0.1%; Ashkenazi 0.2%; Finnish 0.0%; Middle Eastern 0.0%; Amish 0.0%.
Estimated GA genotype frequency.
Global < 0.1%; European < 0.1%; African < 0.1%; East Asian 0.0%; South Asian < 0.1%; Americas < 0.1%; Ashkenazi < 0.1%; Finnish 0.0%; Middle Eastern 0.0%; Amish 0.0%.
Estimated AA genotype frequency.
Showing effects for G/G genotype
Rare Genetic Disorders
Inherited Metabolic Disorders
Hartnup Disease. Evidence: 7/10. Source: PMID 15286788. Typical Risk.
Hyperglycinuria. Evidence: 7/10. Source: PMID 15286788. Typical Levels.
Iminoglycinuria. Evidence: 7/10. Source: PMID 15286788. Typical Risk.
Organ Systems
Kidney Function & Disease
Kidney Function. Evidence: 8/10. Source: PMID 33462484. Typical eGFR.
Blood Creatinine Levels. Evidence: 10/10. Source: PMID 33462484. Typical Levels.
Nutrition & Metabolism
Blood Chemistry & Metabolites
N-delta-Acetylornithine Levels. Evidence: 8/10. Source: PMID 35050183. Typical Levels.
Showing effects for A/A genotype
Hartnup Disease. Evidence: 7/10. Source: PMID 15286788. High Risk.
Hyperglycinuria. Evidence: 7/10. Source: PMID 15286788. Elevated Urine Glycine.
Iminoglycinuria. Evidence: 7/10. Source: PMID 15286788. High Risk.
Kidney Function. Evidence: 8/10. Source: PMID 33462484. Higher eGFR.
Blood Creatinine Levels. Evidence: 10/10. Source: PMID 33462484. Lower Levels.
N-delta-Acetylornithine Levels. Evidence: 8/10. Source: PMID 35050183. Lower Levels.
Showing effects for G/A genotype
Hartnup Disease. Evidence: 7/10. Source: PMID 15286788. Healthy Carrier.
Iminoglycinuria. Evidence: 7/10. Source: PMID 15286788. Healthy Carrier.
Kidney Function. Evidence: 8/10. Source: PMID 33462484. Slightly Higher eGFR.
Blood Creatinine Levels. Evidence: 10/10. Source: PMID 33462484. Slightly Lower Levels.
N-delta-Acetylornithine Levels. Evidence: 8/10. Source: PMID 35050183. Slightly Lower Levels.
What this genotype means for the people who carry it
Sample genotype: GG (Homozygous)
Normal cellular transport of neutral amino acids is maintained by this typical genotype [1]. The B0AT1 transporter proteins function properly in both the kidneys and the intestines, ensuring that vital nutrients like tryptophan are successfully absorbed and reabsorbed [1]. Consequently, there is no risk of developing Hartnup disease, hyperglycinuria, or iminoglycinuria from this locus [2]. Kidney filtration markers, such as creatinine and cystatin C, typically remain within normal baseline ranges [3]. Because amino acid reabsorption is efficient, the body can readily synthesize essential compounds like niacin without requiring high-dose supplementation [1]. Maintaining a standard, balanced diet is sufficient to support overall metabolic and systemic health.
Fully functional B0AT1 transporter protein; Typical urinary amino acid excretion; No genetic predisposition to Hartnup disease.
Sample genotype: AA (Homozygous)
Severe impairment of neutral amino acid transport in both the kidneys and intestines occurs with this genotype [1]. The body is unable to properly reabsorb essential amino acids, resulting in their massive loss through the urine, a condition known as aminoaciduria [1]. This transport failure particularly affects tryptophan, leading to a deficiency in niacin (vitamin B3) synthesis [1]. While many individuals with this genotype remain symptom-free due to a well-balanced diet, physical triggers like fever, stress, or poor nutrition can provoke pellagra-like skin rashes and temporary neurological issues [2]. Clinical evaluations often reveal distinct metabolic profiles, including altered blood creatinine and elevated estimated glomerular filtration rates [3]. Managing this condition typically involves maintaining a high-protein diet and avoiding prolonged sun exposure [2]. Supplementation with niacin or nicotinamide is highly effective at preventing and treating symptomatic episodes [1].
High risk of developing Hartnup disease; Marked loss of neutral amino acids in urine; Increased susceptibility to niacin deficiency under stress.
Sample genotype: GA (Heterozygous)
Carrying a single copy of the A allele reduces the overall efficiency of neutral amino acid transport in the kidneys [1]. Although this single mutation is not enough to cause full-blown Hartnup disease, it is sufficient to trigger hyperglycinuria, a dominant trait characterized by excess glycine in the urine [2]. Most carriers lead completely healthy lives without any noticeable physical symptoms [3]. In rare instances of extreme physical stress or severe nutritional deficiency, mild skin or neurological symptoms could theoretically occur, though this is highly unusual [1]. Kidney function biomarkers may show a slight shift, with minor changes in estimated glomerular filtration rate and blood creatinine levels [4]. No special medical interventions are typically required for carriers. Ensuring a diet rich in protein and B vitamins is generally recommended to maintain optimal metabolic balance.
Carrier of recessive Hartnup disease; Predisposition to dominant hyperglycinuria; Slightly altered kidney biomarker levels.
Variants with shared genetic architecture
rs10224210 · PRKAG2 Gene · Chr 7 · Kidney Health and Gout
rs1047891 · CPS1 Gene · Chr 2 · Urea Cycle and Kidney Health
rs12509595 · FGF5 Gene · Chr 4 · Blood Pressure Risk
Persistence of the common Hartnup disease D173N allele in populations of European origin.
Further evidence for allelic heterogeneity in Hartnup disorder.
Genetics of 35 blood and urine biomarkers in the UK Biobank.
Content updated August 2026.
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