Patients and Public

Everything you need to know about genomic testing

Genomics Overview

What is DNA?

DNA is a chemical that contains our genetic information. It is a long molecule made up of four different chemicals, or bases, represented by the letters A, T, C and G. These bases form long strings which twist around each other to give the characteristic double helix shape.

As well as being helix-shaped, DNA is tightly packed so it takes up less space. If you stretched the DNA in one cell all the way out, it would be about 2 metres long.

The information encoded within DNA contains the instructions for making and maintaining you.

If you are unsure about any of the terms used, please see this  A-Z Genomics glossary .  

What is a genome?

Your genome is all 3.2 billion letters of your DNA. It contains around 20,000 genes. Genes are the instructions for making the different components of our bodies – from the keratin in hair and fingernails to the antibody proteins that fight infection.

Genes make up about 1-5% of your genome. The rest of the DNA, between the genes, is important for controlling the genes. For example, it can switch genes on and off at the right time.

We share over 99% of our genome with other people, but that still leaves more than 3 million differences between us.

Many of these differences in our DNA, known as variants, have no effect, but there are some variants that can affect our health.Many of these differences in our DNA, known as variants, have no impact, but there are some that can affect our health.

To learn more about what a genome is please see the following video:
My genome sequence part 1 and part 2

What is genome sequencing?

Sequencing is a technique that is used to ‘read’ DNA. It finds the order of the letters of DNA (A, T, C and G), one by one, and therefore enables us to find the sequence of someone’s unique 3 billion letters of DNA.

The North Thames team at Great Ormond Street have developed two short videos explaining what a genome sequence is and what happens when a genome is sequenced. If you can’t access YouTube, try watching on Vimeo here and here.

  • Versions of these videos with Turkish sub-titles can also be found here and here (Bu videoların Türkçe çevirileri olan versiyonlarına burada ve burada da bulunabilir).
  • Versions of these videos with Bengali sub-titles can also be found here and here (bangla anubadshah ei videor sanskaranakhan and akhane pawa zabe, বাংলা অনুবাদসহ এই ভিডিওর সংস্করণএখানে ও এখানে পাওয়া যাবে)
  • Versions of these videos translated into Simplified Chinese can be found here and here (这些视频翻译成简体中文的版本可以在这里找到, zhè xiē shì pín fān yì chéng jiǎn tǐ zhōng wén de bǎn běn kě yǐ zài zhè lǐ zhǎo dào)
  • Versions of these videos translated into Traditional Chinese can be found here and here (這些視訊翻譯成傳統中文的版本可以在這裡找到, zhè xiē shì pín fān yì chéng chuán tǒng zhōng wén de bǎn běn kě yǐ zài zhè lǐ zhǎo dào)

Genomic Medicine Service

The 100,000 Genomes project paved the way for a new genomic medicine service (GMS) for the NHS – transforming patient care and enabling equity of access to advanced diagnosis and personalised treatments across England.

What does this new national genomic medicine service mean for patients?

  • Patients living anywhere in England will have the same access to testing
  • The approach to testing will be standardised so that all patients get the best possible care
  • There is a National Genomic Test Directory which details which tests are available and for who in the NHS
  • Because the service and tests offered are regularly reviewed, new technologies and tests will be introduced more rapidly to improve diagnosis
  • As research is a key part of the new service we will be more able to evaluate and introduce new genomic tests in the future to keep up with the latest research and developments

It is also hoped that the benefits of this kind of testing, over time, will result in:

  • Faster diagnosis
  • Personalised medicine with treatments that best suit the patient
  • Improved cancer survival

Genomic Testing

Testing our genomes can help  identify the causes of genetic diseases, and plays a part in the prediction, prevention and treatment of certain conditions. Some examples of how genomic testing can be used to improve healthcare are:

Diagnostic testing:

  • To confirm a diagnosis of a genetic condition.
  • Some rare diseases are caused by as little as a single base change, like a spelling mistake, in someone’s DNA. Looking at the genome of a person affected by a rare disease can help find which DNA changes might be causing the problem.

Predictive testing:

  • To assess your risks of developing a genetic condition and to guide preventative care.
  • For example, if your family has a known history of a genetic condition (such as a hereditary cancer syndrome, or certain inherited heart conditions) you may be offered predictive testing (before you develop any symptoms). If you are then found to carry a genetic variant that increases your risk of a condition, you can be enrolled in the appropriate screening programs to detect any signs of the condition as early as possible or be referred for discussion of preventative measures.

Treatment decisions:

  • For some patients, knowing more about their genome may mean that a particular treatment can be recommended.
  • Genetic testing, in some circumstances, can also be used to predict for each individual which medications are likely to be most effective and cause the least side-effects (also known as pharmacogenomics).

Tumour testing:

In cancer, the tumour cells have developed a different genome to the healthy cells. Comparing the normal and cancer genomes may give clues about ways to treat the cancer. Find out more about genomics and cancer here.

Carrier Testing:

  • This may be available to multiple members of your family if there is a risk of passing specific genetic conditions onto children.
  • Examples of conditions where carrier testing is performed include cystic fibrosis or thalassaemia.

 Prenatal testing:

  • Genetic tests can be used to diagnose conditions in your unborn baby. These might include Down’s syndrome or other conditions known to affect your family. They may also be used to diagnose genetic conditions if unexpected problems are detected on prenatal ultrasound scans.
What to expect from genomic testing

Before you have a genomic test, a member of your clinical team will have a discussion with you about what it might tell you, how long it will take, how you will receive the results and what implications it might have for you and your family. Formally signing a consent form may or may not be required depending upon the type of test you have. For whole genome sequencing and most other forms of sequencing tests we do usually ask you to sign a record of the discussion describing everything that was discussed with you. We will offer you a copy of this to help you remember what the test is and what information you may get.


Genomics is a rapidly developing area. By using data from lots of patients with similar conditions we can learn about rare diseases and cancer, how they develop and then how we might treat them. To do this we will ask patients having genomic tests if they would make their data available for researchers. 
Your data will be anonymised before it is used for research, meaning that any information that can link the data sample to you (, address, email or phone number) will be removed.
Your anonymised clinical and genomic data will only be used for research if you give your consent for this. If you decide not to enter your data into research, you can still have the genomic test and this decision will not affect your clinical treatment. You will also be asked for your consent to be contacted regarding relevant research projects, where your anonymised data may be made available to clinicians leading UK clinical trials of new medicines and tests.


Genomic testing requires samples of your blood, saliva, or tissue, with the type of sample taken depending upon the condition you have and the type of test being carried out.

Further Details:
Rare Disease testing
Cancer testing


The data from your genomic test(s) will be analysed by scientists and shared with your clinical team.

The time it will take for you to get your results will vary depending on the specific type of test that is being carried out. Your doctor will give you an idea of how long it will be before the results are available.

Once the results are available your doctor will arrange to discuss the findings with you and any treatment plans and options. Depending on the results, you may also be referred for genetic counselling.