Blood cells: These develop from stem cells and are formed in the bone marrow. There are three types: red and white blood cells and platelets.

  • White blood cells: These fight infection and disease and there are several types, including neutrophils and lymphocytes, which all play different roles in helping your body fight infections.
  • Red blood cells: Carry oxygen around the body in the bloodstream.
  • Platelets: These help your blood to clot.

Bone marrow: The soft, spongy substance that fills the inner cavities of the bone. It’s where white blood cells, red blood cells and platelets are formed and it also contains fluid, blood vessels and fat.

Calreticulin (CALR) mutation: The calreticulin (CALR) gene was discovered in 2013 and while it is not yet fully understood, it is known that mutations in the CALR gene lead to too many platelets being produced. About 24% of people with MF and 30% of people with ET will have the CALR gene mutation.

Hemoglobin: The protein in red blood cells that carries oxygen.

JAK pathway: The Janus Kinase pathway, to give it its full name, is a chain of signals linked to the production of blood cells. If the chain goes wrong, it’s known as a dysregulated JAK pathway and leads to MPNs.

JAK2 molecule: A molecule present in everyone’s body, which passes messages within a cell.

JAK2V617F mutation: A change in the JAK2 molecule, which triggers a dysregulated JAK pathway. The JAK2V617F mutation is found in 90-95% of people with polycythemia vera (PV), and in around 60% of those with essential thrombocythemia (ET) and 50% to 60% of people with primary myelofibrosis (MF).

MPL mutation: A change in the MPL gene that triggers stem cells to start producing platelets even when levels of platelets in the blood are not low, resulting in too many platelets. Between 5 and 10% of patients with MF and 5% of people with ET have an MPL gene mutation.

Philadelphia chromosome: A chromosomal abnormality found in people with chronic myeloid leukaemia (CML), a related condition. As PV, ET and MF are Philadelphia chromosome-negative, its absence will help your doctor diagnose the condition you have.

Stem cells: Immature cells that can develop in the bone marrow. Stem cells are able to develop into many different cell types, including blood cells. They can keep dividing, developing into mature blood cells to replenish cells in the body.