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The TSC1 and TSC2 genes, and why they matter | CION Cancer Clinics
TSC1 and TSC2 are two genes that work together as a brake on cell growth. When one of them is faulty from birth, cells in many organs can grow too large and form growths, most of them benign. This causes tuberous sclerosis complex, which can affect the brain, skin, kidneys, heart, lungs and eyes. This page explains what the genes normally do, what goes wrong and why a cancer centre follows it. At CION Cancer Clinics, our oncologists explain what a gene result means for you and your family, and plan the checks that follow.
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Medically reviewed by Dr. Naresh GunduConsultant Medical Oncologist · MBBS, DNB (Internal Medicine), DM (Medical Oncology, AIIMS) · last reviewed September 2026, next review due September 2027
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On this page
- What do the TSC1 and TSC2 genes do?
- Which parts of the body can be affected?
- How does one faulty gene lead to a growth?
- The words you will meet, in plain language
- TSC1 or TSC2: what is the difference?
- What this page cannot tell you
- Four things families tell us, and what is actually true
- Common questions about the TSC1 and TSC2 genes
The short answer
What do the TSC1 and TSC2 genes do?
They make two proteins, hamartin and tuberin, that join together and act as a brake on cell growth. They tell a cell when it has grown enough and should stop. When either gene is faulty from birth, the brake is weakened, and cells in several organs can grow larger and multiply more than they should.
Why a weak brake causes growths
The brake works through a growth switch inside the cell called mTOR. With the brake weakened, mTOR stays switched on too often. Cells grow larger and form lumps called hamartomas, made of the organ's own tissue in a disorganised pattern. Most of these growths are benign, meaning they do not spread to other parts of the body.
Why a cancer centre follows it
Most growths in tuberous sclerosis complex are not cancer. But the kidney growths can become large and bleed, a small number of kidney tumours are cancers, and some medicines that calm the mTOR switch are also used in cancer care. That overlap is why oncologists are often part of the team, alongside neurologists, kidney specialists and skin doctors.
Tuberous sclerosis complex is lifelong, but most of its growths are benign. Having it is not the same as having cancer.Where it shows
Which parts of the body can be affected?
The same weak brake acts in every organ, so the condition can show up in very different places, even within one family.
Brain
Small patches of unusual tissue can cause seizures, often starting in infancy. Learning difficulties, autism and behaviour changes are common. A slow-growing growth near the fluid spaces of the brain is watched with scans.
Kidneys
Benign growths made of fat, muscle and blood vessels are very common and can bleed if they grow large. Cysts also appear. A small number of kidney tumours in TSC are cancers, which is why kidneys are checked regularly.
Skin
Often the first visible sign, and the one families notice.
Usually looks like
- Pale, leaf-shaped patches from early childhood
- Small red bumps across the nose and cheeks
- A thickened, rough patch on the lower back
- Small growths around the fingernails or toenails
Heart, lungs and eyes
Heart growths are often seen on pregnancy scans and usually shrink on their own after birth. Some adult women develop a lung condition called LAM. Harmless patches on the retina are common and rarely affect sight.
Not sure whether this applies to you?
Ask an oncologistHow a fault turns into a growth
How does one faulty gene lead to a growth?
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You carry two copies of each gene
One copy of TSC1 and one of TSC2 came from each parent. One working copy is usually enough to keep the brake on.
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A fault knocks out one copy from the start
Sometimes the fault is inherited from a parent. In about two in three people with TSC it is new, and neither parent carries it. Either way it is present in every cell from birth.
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The second copy is lost in some cells
During life, the remaining working copy can be damaged in a single cell. Now that cell has no working brake at all.
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The growth switch stays on
Without the brake, mTOR keeps telling the cell to grow. That cell and its offspring form a hamartoma in whichever organ it sits.
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Why the same fault looks different in each person
Where and when the second copy is lost is largely chance. That is why two relatives with the same fault can be affected very differently.
On your report
The words you will meet, in plain language
- Hamartoma
- A benign lump made of the organ's own tissue growing in a disorganised way. It is the typical growth of TSC.
- Angiomyolipoma
- A benign kidney growth made of blood vessels, muscle and fat. Large ones can bleed and are treated before that happens.
- SEGA
- A slow-growing, benign brain growth near the fluid spaces. It is watched because it can block the flow of fluid if it enlarges.
- LAM
- A lung condition, mostly in women with TSC, in which abnormal muscle cells form small cysts in the lungs.
- mTOR inhibitor
- A medicine that calms the overactive growth switch. Some can shrink TSC growths in the brain, kidney and lung.
- Germline
- Present in every cell from birth, and therefore inheritable. A change found only inside a tumour is called somatic.
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Side by side
TSC1 or TSC2: what is the difference?
A TSC1 fault
A TSC2 fault
Makes a faulty hamartin protein
Makes a faulty tuberin protein
Found in fewer families
Found in most families with a known fault
Often milder, on average
Often more severe, on average
Passed on in the same way
A large deletion can also affect a nearby kidney cyst gene
Being straight with you
What this page cannot tell you
It cannot tell you what a particular result means for you or your child. The same gene can be affected in many different ways. What your specific variant means is a question for the counsellor who ordered the test.
It cannot predict how TSC will affect one person
Two relatives with the same fault can have very different lives. One may have seizures from infancy, another only a few skin patches noticed in adulthood. Doctors cannot yet predict severity from the gene result alone. Studies so far give averages across many people, not forecasts for one.
Who this does not apply to
This page is about faults present from birth. Some cancers, including certain bladder and kidney cancers, pick up TSC1 or TSC2 changes inside the tumour only. Those changes are not inherited, and they are covered under targeted therapy. Most people who see a TSC change on a tumour report do not have tuberous sclerosis.
If you are unsure whether a report refers to an inherited fault or a tumour change, call the helpline and ask.Commonly believed
Four things families tell us, and what is actually true
"Tuberous sclerosis is a kind of cancer."
It is a genetic condition that causes mostly benign growths. A small number of kidney tumours in TSC are cancers, which is why the kidneys are watched, but most people with TSC never develop cancer.
"Neither parent has it, so it cannot be genetic."
Most people with TSC are the first in their family. The fault appeared new in them. It is still genetic, and they can pass it on to their own children.
"The seizures and the skin marks are separate problems."
They come from the same weak brake acting in different organs. That is why one person may need a neurologist, a skin doctor and a kidney specialist, and why care works best when they talk to each other.
"My TSC is mild, so my children's will be mild too."
Severity can differ a great deal between a parent and a child with the same fault. Each child has a one in two chance of inheriting it, and a child may be affected more or less than the parent.
Questions we are asked
Common questions about the TSC1 and TSC2 genes
Is tuberous sclerosis inherited?
It can be. A parent with a TSC1 or TSC2 fault has a one in two chance of passing it to each child. But in about two in three people with TSC, the fault is new and neither parent carries it. Parents are usually offered testing to check which applies.
How is tuberous sclerosis diagnosed?
Often from signs such as seizures in a baby, heart growths on a pregnancy scan, skin patches or kidney growths. A genetic test can confirm it. Some people with a clear diagnosis have no fault found on standard testing, and the diagnosis still stands.
Does a TSC fault mean I will get cancer?
No. Most TSC growths are benign. The kidneys carry a small raised chance of cancer, which is one reason kidney scans are part of lifelong care. Your team will explain which checks you need and how often.
What are mTOR inhibitors, and who needs them?
They are medicines, such as everolimus and sirolimus, that calm the overactive growth switch. They can shrink some brain, kidney and lung growths in TSC, and some are also used in cancer care. Specialists prescribe them and monitor them with regular blood tests.
Can a parent have TSC without knowing?
Yes. Some adults have only faint skin patches or a kidney growth found by chance. Occasionally a parent carries the fault in only some of their cells. A careful examination and, where needed, a test can clarify this.
Should my child be tested?
If a parent carries a known fault, testing a young child is often advised. Some TSC problems, such as seizures, can begin in infancy, and early recognition helps. Your counsellor will explain the timing that suits your family.
My tumour report mentions TSC2. Do I have tuberous sclerosis?
Usually not. A change found only in tumour tissue is not inherited and may guide cancer treatment instead. If there are other signs of TSC, or the laboratory suggests it, a blood test for an inherited fault can be arranged through a counsellor.
Where do I start?
Bring any genetic report, scan reports and a list of family members with seizures, skin marks or kidney growths. A genetic counsellor can explain what the result means. Call the CION helpline if you are not sure who to approach.
Your Specialists
Meet CION's oncologists. Bring your family history or genetic report to them.
Our medical oncologists see people with a strong family history of cancer, arrange genetic counselling and testing where it fits, and plan the checks that follow.
Medical Oncologist
Dr. C. Raghavendra Reddy
MBBS(Gold Medal), DNB(General Medicine), DM(Medical Oncology)(Gold Medal)
Medical Oncologist
Dr. Bharati Devi Gorantla
MBBS, MD(General Medicine), DM(Medical Oncology)(Adyar,Chennai), ECMO, MRCP SCE(UK)
Medical Oncologist
Dr. Owais Mohammed
MBBS, MD (General Medicine), DrNB (Medical Oncology), ECMO, MRCP SCE (Medical Oncology) (UK)
Medical Oncologist
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Sources
- GeneReviews (NCBI) — Tuberous Sclerosis Complex
- MedlinePlus Genetics — Tuberous sclerosis complex
- MedlinePlus Genetics — TSC2 gene
- National Cancer Institute — Genetics of Kidney Cancer (Renal Cell Cancer) (PDQ)
This page is general information, not a prescription. Do not change or stop any treatment based on what you read here. If anything is worrying you, contact your own treating team — or call our helpline and we will help you reach the right specialist.
Keep reading
Related pages
TSC1 and TSC2: which cancers and what riskThe kidney and other risks, set out plainlyTuberous sclerosis and kidney tumoursWhy kidney growths are watched, and when they are treatedTesting the family for TSC1 and TSC2Who should be tested, and in what orderSurveillance for TSC1 and TSC2 carriersThe checks a carrier needs through life
Talk to us
Have a TSC1 or TSC2 result you do not understand?
Bring the report and any scan results. We will arrange for a genetic counsellor to explain what it means for you and your family, in plain language. One helpline serves every CION centre.