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Essential thrombocythaemia (ET)

We're here for you if you want to talk

0808 2080 888

[email protected]

What is essential thrombocythaemia (ET)?

ET is a type of myeloproliferative neoplasm (MPN). If you have it, your bone marrow (the soft spongy material inside some of your bones) makes too many platelets. ET usually develops very slowly and for the majority of people it doesn’t affect their normal lifespan.

What causes ET?

All cells in your body contain a set of instructions that tell the cell what to do and when to do it, stored inside the cells in structures called chromosomes. The chromosomes are made up of a chemical known as DNA.

The DNA is arranged in sections called genes. There are 23 pairs of chromosomes in each cell in your body. When cells divide to form new cells, normally the chromosomes stay the same in each new cell.

However with ET, something goes wrong and causes a genetic fault to occur: you may hear your doctor talk about a fault, or mutation, in the JAK2, CALR or MPL genes. Genetic faults may happen because you’ve been exposed to hazardous chemicals, but more usually because of copying mistake when a cell was dividing. Around 60% of people with ET have the JAK2 genetic fault, 30% have the CALR fault and 5% have the MPL fault.

The JAK2 and MPL genes are involved in the response of bone marrow stem cells to different growth factors. A growth factor is a substance which sends signals to your stem cells, so they can produce the right number of blood cells to keep you healthy. A growth factor is released when there are low levels of platelets in the blood.

When you have a fault with your JAK2 or MPL gene, the stem cells can start producing platelets even when they’ve not been ‘told’ to do so by the growth factor. This results in too many platelets being produced.

The CALR gene was discovered in 2013. We don’t fully understand it yet, but we know it causes signals that lead to too many platelets being produced.

The presence of one of these genes in your blood might lead doctors diagnosing you with MPN. However, we‘re learning about these faulty genes, and their impact on treatment options and prognosis, all the time. If you’d like to know more about those genes, your consultant will be happy to talk to you about them.

It’s important to note that these genetic faults happen during a person’s lifetime. As you’re not born with these faults, you can’t pass these onto your children.

As well as the presence of certain genetic faults, there are other factors which might lead to a higher risk of getting ET.


People who get ET are usually between 50 and 70 years old. The condition is rare in children, but can occur at any age.


Men and women are at equal risk of developing ET.

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