This study aims to develop a systematic genetic screening strategy for (potential) female carriers of haemophilia by identifying as many female carriers as possible within the families of haemophilia patients regularly followed at Cliniques universitaires Saint-Luc (CUSL) and to search for differences between female carriers of haemophilia A (HA) and B (HB).

In order to complete our local registry of female carriers, the family trees of haemophilia patients will be systematically updated during their follow-up consultations at the haemophilia centre. Female carriers not yet known in our centre, identified by the updating of pedigrees, will be invited to present themselves in the haematology consultation and to participate in the study by means of an invitation and information letter which will be given/sent to them by the index haemophilia patient. Female carriers already known for whom missing data and/or the indication of regular follow-up have been identified during the file review will also be invited to attend a consultation within the framework of the study.

At these consultations, (potential) carriers will be given information about haemophilia, the mode of genetic transmission and the implications of carrier status on patients' lives (bleeding prevention, reproductive choices, current haemophilia treatments and future prospects). We will then determine the bleeding phenotype of each patient by taking a comprehensive bleeding history. With the consent of the participants concerned, the familial genetic variant responsible for haemophilia will be sought in them in order to definitively establish their carrier status. The basal level of coagulation factors VIII (HA)/IX (HB) will also be determined. If a deficiency is found, haemostatic treatment adapted to the patient's situation will be initiated and clinical follow-up outside the study recommended.

After completion of the data collection, the data will be analyzed and compared between HA and HB carriers in order to identify possible differences between these two populations.

Inclusion Criteria:

- Family member affected by haemophilia A or B and potential/obligate carriership of HA/HB.

Exclusion Criteria:

• None.

1. Carriers of HA/HB already known and registered at the HTC (i.e. with an EMR).
2. Carriers identified by pedigree review, invited/informed by family related haemophilia patients registered in the HTC, accepting to participate after informed consent.