International multicenter retrospective observational study consisting of two parts: a nested case-control study (part A) and a comparative retrospective cohort study (part B).

The study will include ET patients (diagnosed according to "International consensus classification of myeloid neoplasms and acute leukemias - 2022" criteria) who are JAK2V617F positive and have progressed to PV (diagnosed according to "International consensus classification of myeloid neoplasms and acute leukemias - 2022" criteria) by 31/12/2020. The date of PV diagnosis will be defined as the index date and this group of patients will be defined as ET-to-PV cases.

Part A - Nested case-control study Controls for ET-to-PV cases will be patients diagnosed with ET, according to the "International consensus classification of myeloid neoplasms and acute leukemias - 2022" criteria, JAK2V617F positive who did not progress to PV after a comparable period of time. This control group will be selected using a 1:1 matched case-control design.

For each patient who progressed to PV (case), a control will be selected with the following characteristics:

• year of ET diagnosis (+/-2 years as compared to the date of the case);
• age at ET diagnosis (+/-3 years as compared to the age of the case);
• duration of ET disease (+/-2 years as compared to the period between the diagnosis of ET and the index date of the respective case).

Part B - Comparative retrospective cohort study

Patients diagnosed with de novo PV (no progression from previous ET) according to the "International consensus classification of myeloid neoplasms and acute leukemias - 2022" data criteria, will be recruited using a 1:1 matching procedure according to the following characteristics:

• year of PV diagnosis (+/-2 years as compared to the date of the case);
• age at PV diagnosis (+/-3 years as compared to the age of the case);
• duration of PV disease (+/-2 years as compared to the period between PV diagnosis and the last follow-up of the respective case).

Furthermore, to have at least 5 years of follow-up, de novo PV patients must be diagnosed no later than 2020.

Inclusion Criteria:

- ET patients (diagnosed according to "International consensus classification of myeloid neoplasms and acute leukemias - 2022" criteria) who are JAK2V617F positive and have progressed to PV (diagnosed according to "International consensus classification of myeloid neoplasms and acute leukemias - 2022" criteria) by 31/12/2020. The date of PV diagnosis will be defined as the index date and this group of patients will be defined as ET-to-PV cases.

Exclusion Criteria:

• None.

Nested case-control study: the prevalence of at least one of the following proliferative conditions in the control group, a neutrophil/lymphocyte ratio (N/L ratio) above the median, an LDH above the median, being JAK2V617F homozygous and having splenomegaly.

Comparative retrospective cohort study: it is known that the incidence of the composite outcome (vascular events, cancers, progression to myelofibrosis, progression to acute myeloid leukemia, death) in PV de novo patients.