Mild phenylketonuria | About the Disease

Disease Information

Summary

Mild phenylketonuria is a rare form of phenylketouria (PKU variant), an inborn error of amino acid metabolism, characterized by symptoms of PKU of mild to moderate severity. Patients with blood phenylalanine concentrations of 600-1,200 micromol/L are considered to have mild PKU. Clinical signs include reduced cognitive function and behavioral and developmental disorders. It is caused by certain genetic changes in the PAH gene which result in slightly higher activity of the phenylalanine hydroxylase compared with the classic phenylketonuria where there is a complete or near-complete deficiency of phenylalanine hydroxylase activity. Inheritance is autosomal recessive.

About Mild phenylketonuria

Many rare diseases have limited information. Currently, GARD aims to provide the following information for this disease:

Symptoms:GARD does not currently have information about when this disease may start to appear.
Cause:GARD does not currently have information about the cause of this disease.
Organizations:Contact a GARD Information Specialist to help search for patient organizations dedicated to this rare disease.

Resource(s) for Medical Professionals and Scientists on This Disease:

ClinGen provides expert-reviewed information on genes and gene variants for use in research and medicine.
MONDO provides GARD with information for this disease.
RARe-SOURCE™ offers rare disease gene variant annotations and links to rare disease gene literature.

Resource(s) for Medical Professionals and Scientists on This Disease:

ClinGen provides expert-reviewed information on genes and gene variants for use in research and medicine.
MONDO provides GARD with information for this disease.
RARe-SOURCE™ offers rare disease gene variant annotations and links to rare disease gene literature.

About Mild phenylketonuria

Many rare diseases have limited information. Currently, GARD aims to provide the following information for this disease:

Symptoms:GARD does not currently have information about when this disease may start to appear.
Cause:GARD does not currently have information about the cause of this disease.
Organizations:Contact a GARD Information Specialist to help search for patient organizations dedicated to this rare disease.

Can diseases be passed down from parent to child?

A biological parent can sometimes pass down genetic changes, called mutations, that cause a disease or increase the chances of developing it. This is called inheritance. Knowing if other family members have had the disease, also known as your family health history, can give your medical team important information. The Surgeon General offers a tool to help you collect your family health history.

Autosomal Recessive

Autosomal means the gene involved is located on one of the numbered chromosomes. Recessive means that a child must inherit two copies of the mutated gene, one from each biological parent, to be affected by the disease. A carrier is a person who only has one copy of the genetic mutation. A carrier usually doesn't show any symptoms of the disease.

If both biological parents are carriers, there is a 25% their child inherits both copies of the mutated gene and is affected by the disease. Additionally, there is a 50% chance their child inherits only one copy of the mutated gene and is a carrier.

Learn more about inheritance patterns from the National Library of Medicine (NLM).

Autosomal Recessive

Navigating Health Care Decisions

On average, it can take more than six years to receive an accurate diagnosis. Many primary care providers (PCPs) may not be familiar with rare diseases, and patients often need to visit multiple specialists or seek second opinions to get answers.

If a diagnosis remains unclear, visiting a multidisciplinary care center or university hospital may help. These centers bring together teams of specialists who can work together to evaluate symptoms and coordinate a diagnosis. This team-based approach is also helpful after a diagnosis, when managing care for rare diseases.

Because only about 5% of rare diseases have FDA-approved treatments, finding the right healthcare team to manage your symptoms and overall health is essential. People living with rare diseases often face challenges such as delayed diagnosis, limited treatment options, and difficulty accessing knowledgeable providers. Building a care team that understands your needs can make a significant difference in your quality of life.

Your Health Care Team

Why is building the right health care team important?

Building the right health care team is key to the diagnosis, treatment, and management of your long-term health journey living with a rare disease. Start by choosing a primary care provider (PCP). Your PCP will be your main point of contact and help coordinate care with other medical professionals. Your PCP may order tests or refer you to specialists. To find a PCP near you, use the Medicare provider search tool and enter your location and “Primary Care Provider.”

Seeing multiple specialists is important for people with rare diseases because these conditions often affect many parts of the body and require care from doctors with different expertise. Most primary care providers may not be familiar with rare diseases, so involving specialists can lead to a more accurate diagnosis and better care. A coordinated team approach ensures that all symptoms are addressed and that care is well-managed. It can also connect patients with the latest research or treatment options.

Multidisciplinary Care Centers

Is It Time to Get a Second Opinion or Specialized Evaluation?

If you've visited your PCP, met with specialists, and undergone the recommended tests, but are still searching for a diagnosis, it may be time to visit an academic medical center or, for pediatric patients, a children's hospital. Academic medical centers and children's hospitals, often called multidisciplinary care centers, typically bring together specialists from different fields to work together on complex cases like rare diseases.

Multidisciplinary care centers may offer more coordinated care and be involved in clinical research, which may help reduce the time to diagnosis and provide access to emerging diagnostic tools. Specialists at these centers may have a deeper understanding of rare diseases and serve as a resource when you'd like a second opinion, particularly when test results or treatment plans are not delivering expected results.

Find hospitals that may partner with medical schools and programs in your area.

Rare Disease Experts

How can you find a rare disease expert?

If a diagnosis, care management, or treatment plan remains unclear despite extensive efforts by your PCP and specialists, it may be time to find a rare disease expert for your disease, if available. A rare disease expert is a medical provider that has knowledge or training on specific rare disease(s), but there may only be a few experts in your state, region, or country. Rare disease experts may work at large research or teaching hospitals, sometimes called centers of excellence. Centers of Excellence commit to sharing knowledge and best practices that can lead to improved care and treatment for individuals living with a rare disease.

You can also contact a GARD Information Specialist for help finding experts, centers of excellence, or clinics that focus on your disease.

You can ask your care providers for help finding an expert or use directory tools to search for experts near you. The National Organization for Rare Disorders (NORD) maintains a list of rare disease centers with experts that work together to find treatments and cures for a broad range of rare diseases. Search NORD's Center Directory to find experts near you.

Find Your Community

How can patient organizations help?

Patient organizations can help patients and families connect. They build public awareness of the disease and are a driving force behind research to improve patients' lives. They may offer online and in-person resources to help people live well with their disease. Many collaborate with medical experts and researchers.

Services of patient organizations differ, but may include:

Ways to connect to others and share personal stories
Easy-to-read information
Up-to-date treatment and research information
Patient registries
Lists of specialists or specialty centers
Financial aid and travel resources

Please note: GARD provides organizations for informational purposes only and not as an endorsement of their services. Contact a GARD Information Specialist for more information on organizations that may be dedicated to this disease. Please contact an organization directly if you have questions about the information or resources it provides.

View GARD's criteria for including patient organizations, which can be found under the FAQs on our About GARD page. Request an update or to have your organization added to GARD.

Patient Organizations

4 Organizations

Global Genes

People With

Rare Diseases

Helpful Links

Research Registry

Country

United States

National Organization for Rare Disorders

People With

Rare Diseases

Helpful Links

List of Experts

Country

United States

Genetic Alliance

People With

Rare Diseases

Country

United States

RARE Foundation

People With

Rare Diseases

Country

United States

Organization	People With	Helpful Links	Country
Global Genes	Rare Diseases	Research Registry	United States
National Organization for Rare Disorders	Rare Diseases	List of Experts	United States
Genetic Alliance	Rare Diseases		United States
RARE Foundation	Rare Diseases		United States

Participate in Research

Clinical studies are a part of clinical research and play an important role in medical advances for rare diseases. Through clinical studies, researchers may ultimately uncover better ways to treat, prevent, diagnose, and understand human diseases.

What Are Clinical Studies?

Clinical studies are medical research involving people as participants. There are two main types of clinical studies:

Clinical trials determine if a new test or treatment for a disease is effective and safe by comparing groups receiving different tests/treatments.
Observational studies involve recording changes over time among a specific group of people in their natural settings.

Learn more about clinical trials from this National Institutes of Health webpage.

Why Participate in Clinical Studies?

People participate in clinical trials for many reasons. People with a disease may participate to receive the newest possible treatment and additional care from clinical study staff as well as to help others living with the same or a similar disease. Healthy volunteers may participate to help others and to contribute to moving science forward.

To find the right clinical study we recommend you consult your doctors, other trusted medical professionals, and patient organizations. Additionally, you can use ClinicalTrials.gov to search for clinical studies by disease, terms, or location.

What if There Are No Available Clinical Studies?

ResearchMatch helps connect people interested in research studies with researchers from top medical centers across the United States. Anyone from the U.S. can register with this free program funded by NIH. Researchers from participating institutions use the database to search for and invite patients or healthy volunteers who meet their study criteria to participate.

Why may you want to consider joining the All of Us Research Program?

The All of Us Research Program is inviting 1 million people from all backgrounds across the U.S. to help build one of the most diverse health databases in history. Researchers will use the data to learn how our biology, lifestyle, and environment affect health. This may one day help them find ways to treat and prevent diseases.

ClinicalTrials.gov, an affiliate of NIH, provides current information on clinical research studies in the United States and abroad. Talk to a trusted doctor before choosing to participate in any clinical study. We recommend checking this site often and searching for studies with related terms/synonyms to improve results.

Search ClinicalTrials.gov for this disease.

GARDGenetic and Rare Diseases
Information Center

Contact a GARD Information Specialist if you need help finding more information on this rare disease or available clinical studies. Please note that GARD cannot enroll individuals in clinical studies.

Contact GARD

Please allow 2 to 10 business days for us to respond.

Sources & References

Last Updated: September 2026