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PubMed This is a summary of 19 peer-reviewed journal articles Updated
Pediatrics

Building a Foundation: The Biology and Genetics of Crigler-Najjar Syndrome

At a Glance

Crigler-Najjar Syndrome is a rare genetic disorder caused by mutations in the UGT1A1 gene. This defect prevents the liver from processing bilirubin, leading to dangerous buildup. Type 1 is severe and requires phototherapy or transplant, while Type 2 is milder and may respond to medication.

Receiving a diagnosis of Crigler-Najjar Syndrome (CNS) for your child can feel overwhelming. It is an exceptionally rare condition that fundamentally changes how your child’s body processes a natural waste product called bilirubin [1][2]. While the road ahead requires dedicated care, understanding the science behind the diagnosis is the first step in managing your child’s health and advocating for their needs.

What is Crigler-Najjar Syndrome?

Crigler-Najjar Syndrome is a rare genetic disorder that causes high levels of unconjugated bilirubin to build up in the blood [1]. Bilirubin is a yellow pigment created when the body breaks down old red blood cells [3]. In most people, the liver processes this pigment so it can be safely removed from the body. In children with CNS, this process is broken.

The condition is divided into two types based on the severity of the enzyme deficiency:

  • Type 1 (CNS-I): The most severe form, where there is a total or near-total absence of the necessary liver enzyme [4][3].
  • Type 2 (CNS-II): A milder form where the enzyme still has some partial activity [5][3].

The Biology: Why Bilirubin Builds Up

To understand CNS, it helps to look at how a healthy liver works. Normally, an enzyme called UGT1A1 performs a process called glucuronidation [3].

  1. The “Raw” State: Bilirubin starts as “unconjugated” or lipophilic (fat-soluble), meaning it cannot dissolve in water or be easily excreted in bile or urine [6].
  2. The Transformation: The UGT1A1 enzyme attaches a sugar molecule to the bilirubin, making it “conjugated” (water-soluble) [3][7].
  3. The Exit: Once water-soluble, the bilirubin can travel through the bile ducts and leave the body through stool.

In children with CNS, a mutation in the UGT1A1 gene prevents the body from creating this enzyme correctly [1][8]. Because the bilirubin remains fat-soluble, it cannot leave the body. Instead, it accumulates in the blood and can cross the blood-brain barrier, where it may become toxic to the central nervous system [6][3].

Genetics and Inheritance

CNS is an autosomal recessive disorder [2][9]. This means a child must inherit two altered copies of the UGT1A1 gene—one from each parent—to develop the syndrome [2].

  • Carriers: Parents are typically “carriers.” They have one functional gene and one mutated gene. Carriers usually do not show symptoms of CNS, though some may have a very mild form of jaundice known as Gilbert Syndrome [4][10].
  • The Odds: When both parents are carriers, there is a 25% chance with each pregnancy that the child will have CNS, a 50% chance the child will be a carrier, and a 25% chance the child will have two functional genes [11].

Rarity and Current Consensus

CNS is extremely rare, though exact global figures are difficult to pinpoint because it is so infrequent [1][12]. It is found in populations all over the world [4][13].

Medical experts agree on several “stabilizing facts” regarding the condition:

  • Genetic Cause: The 2025 Expert Consensus confirms that CNS is caused by biallelic (both copies) mutations in the UGT1A1 gene [14].
  • Standard of Care: For Type 1, the immediate priority is phototherapy (blue light treatment) to break down bilirubin in the skin, while liver transplantation remains the only definitive cure [2][15].
  • Type 2 Management: Children with Type 2 often respond to a medication called phenobarbital, which can “jumpstart” the small amount of enzyme activity they do have [16][17].

A Note for Parents

Learning your child has a condition that affects only a few hundred people worldwide is a heavy burden to carry. It is normal to feel isolated or overwhelmed by the technical nature of the diagnosis. However, recent advances in gene therapy and mRNA technology are being actively researched, offering hope for new ways to treat the underlying enzyme deficiency in the future [18][19]. You are your child’s best advocate, and understanding these biological foundations is the first step in that journey. To find support and learn more about clinical trials, connecting with rare disease registries and patient advocacy groups can be incredibly empowering [19].

Common questions in this guide

What causes Crigler-Najjar Syndrome?
Crigler-Najjar Syndrome is caused by a mutation in the UGT1A1 gene. This mutation prevents the liver from producing a necessary enzyme, which means the body cannot break down and remove bilirubin safely.
What is the difference between Crigler-Najjar Type 1 and Type 2?
Type 1 is the most severe form, characterized by a complete or near-total absence of the required liver enzyme. Type 2 is a milder form where the enzyme still has partial activity, meaning it often responds to certain medications.
How is Crigler-Najjar Syndrome inherited?
It is an autosomal recessive disorder, meaning a child must inherit two mutated copies of the UGT1A1 gene—one from each parent. Parents are typically carriers who have one functional gene and do not show symptoms of the syndrome themselves.
What are the main treatments for Crigler-Najjar Syndrome?
For Type 1, daily phototherapy (blue light treatment) is the standard care to break down bilirubin, though a liver transplant is the only definitive cure. Children with Type 2 are often managed with a medication called phenobarbital to boost their existing enzyme activity.
Why is a buildup of unconjugated bilirubin dangerous?
Because bilirubin remains fat-soluble instead of becoming water-soluble, it cannot leave the body through stool. If levels get too high, the bilirubin can cross the blood-brain barrier and potentially cause toxic damage to the central nervous system.

Questions to Ask Your Doctor

Curated prompts to bring to your next appointment.

  1. 1.Does my child have Crigler-Najjar Type 1 or Type 2, and what specific UGT1A1 mutations were found in their genetic testing?
  2. 2.What is the current target range for my child's bilirubin levels to prevent neurological damage?
  3. 3.How many hours of phototherapy are recommended daily, and how will we monitor its effectiveness as my child grows?
  4. 4.Is our local hospital equipped to manage a 'bilirubin crisis' if my child becomes ill or dehydrated?
  5. 5.Based on the specific genetic findings, should we be considering a liver transplant evaluation or looking into gene therapy clinical trials?

Questions For You

Tap a prompt to share your answer — we'll use it plus this page's context to start a tailored conversation.

References

References (19)
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    Management of Crigler-Najjar syndrome.

    Tcaciuc E, Podurean M, Tcaciuc A

    Medicine and pharmacy reports 2021; (94(Suppl No 1)):S64-S67 doi:10.15386/mpr-2234.

    PMID: 34527915
  2. 2

    Disease burden of Crigler-Najjar syndrome: Systematic review and future perspectives.

    Dhawan A, Lawlor MW, Mazariegos GV, et al.

    Journal of gastroenterology and hepatology 2020; (35(4)):530-543 doi:10.1111/jgh.14853.

    PMID: 31495946
  3. 3

    Therapeutic Options for Crigler-Najjar Syndrome: A Scoping Review.

    Sambati V, Laudisio S, Motta M, Esposito S

    International journal of molecular sciences 2024; (25(20)) doi:10.3390/ijms252011006.

    PMID: 39456788
  4. 4

    Spectrum of UGT1A1 variants in Pakistani children affected with inherited unconjugated hyperbilirubinemias.

    Zubaida B, Cheema HA, Hashmi MA, Naeem M

    Clinical biochemistry 2019; (69()):30-35 doi:10.1016/j.clinbiochem.2019.05.012.

    PMID: 31145902
  5. 5

    Type II Crigler-Najjar syndrome: a case report and literature review.

    He T, Geng X, Zhu L, et al.

    Frontiers in medicine 2024; (11()):1354514 doi:10.3389/fmed.2024.1354514.

    PMID: 38784231
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    Free Bilirubin Induces Neuro-Inflammation in an Induced Pluripotent Stem Cell-Derived Cortical Organoid Model of Crigler-Najjar Syndrome.

    Pranty AI, Wruck W, Adjaye J

    Cells 2023; (12(18)) doi:10.3390/cells12182277.

    PMID: 37759499
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    A Case of Crigler-Najjar Syndrome Type II During Pregnancy and Its Management.

    Singh S, Tayade S, Makhija N, et al.

    Cureus 2024; (16(4)):e59075 doi:10.7759/cureus.59075.

    PMID: 38800243
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    Crigler Najjar Syndrome Type 2 (CNS Type 2): An Unwonted Cause of Jaundice in Adults.

    Kumar P, Sasmal G, Gupta S, et al.

    Journal of clinical and diagnostic research : JCDR 2017; (11(7)):OD05-OD06 doi:10.7860/JCDR/2017/28195.10221.

    PMID: 28892962
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    Paternal uniparental disomy of chromosome 2 resulting in a concurrent presentation of Crigler-Najjar syndrome type I and long-chain 3-hydroxyacyl-CoA dehydrogenase deficiency.

    Knapp A, Jagła M, Madetko-Talowska A, et al.

    American journal of medical genetics. Part A 2022; (188(6)):1848-1852 doi:10.1002/ajmg.a.62696.

    PMID: 35199468
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    Novel mutations in Uridyl-diphosphate-glucuronosyl-transferase 1A1 (UGT1A1) gene in Tunisian patients with unconjugated hyperbilirubinemia.

    Trabelsi N, Chaouch L, Haddad F, et al.

    European journal of medical genetics 2021; (64(2)):104139 doi:10.1016/j.ejmg.2021.104139.

    PMID: 33421605
  11. 11

    p.Cys223Tyr mutation causing Crigler-Najjar syndrome type II.

    Xiong QF, Zhou H, Yang YF

    JGH open : an open access journal of gastroenterology and hepatology 2020; (4(5)):1009-1011 doi:10.1002/jgh3.12355.

    PMID: 33102778
  12. 12

    [Clinical and pathological features of inherited metabolic liver disease in adults].

    He ZY, You H, Zhao XY

    Zhonghua gan zang bing za zhi = Zhonghua ganzangbing zazhi = Chinese journal of hepatology 2018; (26(12)):889-893 doi:10.3760/cma.j.issn.1007-3418.2018.12.003.

    PMID: 30669779
  13. 13

    Liver Transplantation in a Child With Crigler-Najjar Syndrome Type I: A Case Report With Review of the Literature.

    Alharbi FA, Al-Shammari NR, Aloqeely KM

    Cureus 2023; (15(7)):e42064 doi:10.7759/cureus.42064.

    PMID: 37602038
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    [Expert consensus on the diagnosis and therapy of inherited hyperbilirubinemia (version 2025)].

    Zhonghua gan zang bing za zhi = Zhonghua ganzangbing zazhi = Chinese journal of hepatology 2025; (33(8)):747-759 doi:10.3760/cma.j.cn501113-20250518-00194.

    PMID: 40873074
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    Liver Transplantation Outcomes in Crigler-Najjar Syndrome in Iran: A Single-Center Retrospective Cohort Study Over 20 Years.

    Teimoury S, Beyzaei Z, Shamsaeefar A, et al.

    Health science reports 2026; (9(5)):e72393 doi:10.1002/hsr2.72393.

    PMID: 42057867
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    A novel UGT1A1 gene mutation causing severe unconjugated hyperbilirubinemia: a case report.

    Shi X, Aronson S, Khan AS, Bosma PJ

    BMC pediatrics 2019; (19(1)):173 doi:10.1186/s12887-019-1555-y.

    PMID: 31142299
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    Crigler-Najjar Syndrome Type II Diagnosed in a Patient with Jaundice Since Birth.

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    PMID: 30266131
  18. 18

    Lipid nanoparticle-encapsulated mRNA therapy corrects serum total bilirubin level in Crigler-Najjar syndrome mouse model.

    Greig JA, Chorazeczewski JK, Chowdhary V, et al.

    Molecular therapy. Methods & clinical development 2023; (29()):32-39 doi:10.1016/j.omtm.2023.02.007.

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  19. 19

    Gene Therapy in Patients with the Crigler-Najjar Syndrome.

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    PMID: 37585628

This page provides foundational educational information about the biology and genetics of Crigler-Najjar Syndrome. Always consult a pediatric hepatologist or metabolic specialist for your child's specific medical care and treatment plan.

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