Codons and amino-acids are numbered on this site in two ways. In HGVS numbering, codons are numbered with codon +1 coding for the first residue (Met) of the 46-residue signal peptide/propeptide (this is -46 in Legacy numbering). In Legacy numbering, codon +1 refers to that coding for the first amino-acid of the mature FVIII protein (in HGVS numbering, this is codon +47). HGVS numbering is recommended, however Legacy numbering is extensively used in FIX publications, particularly before the year 2000.

Mutations causing haemophilia B are classified by factor IX clotting activity level into the following groups: severe (≤1%), moderate (1-5%) and mild >5%. Where a laboratory value does not correlate with the severity given in a particular report we have attempted to clarify the discrepancy with the original authors. If the discrepancy remains, the clinical severity of the phenotype as reported by the authors has been used. For variants causing mild haemophilia B we have attempted to indicate whether the deleterious effect of the mutation is predominantly quantitative (reduced levels of a normally functioning protein) or qualitative (dysfunctional protein which may or may not also be associated with a reduction in protein level). This is indicated in results tables under the "Type" heading using the conventions used for other coagulation proteins so that Type I refers to quantitative defects with an activity:antigen ratio >0.7 and Type II are qualitative defects with a ratio ≤ 0.7. These categories broadly correspond respectively to the terms CRM (cross-reacting material) negative and positive sometimes found in older publications.

The information contained on this web site is provided for scientific research purposes only. We do not give medical advice or recommend any particular treatment for specific individuals. Click here for patient information.

The following people worked on the 2023 version of the project:

• Ziqian Xu, UCL, London
• Holly J. Spencer, UCL, London
• Victoria A. Harris, UCL, London
• Stephen J. Perkins, UCL , London

The FIX variants database was created in S.J.Perkins's laboratory at UCL in 2010. For the 2013 version of the database, we are grateful for Dr Peter Green's agreement to use the KCL data as the basis for this updated and upgraded Database: Dr Green at King's College London originated and maintained an extensive database of F9 variants. That work was supported through a medical educational grant from Pfizer UK Ltd, and support from the Special Trustees of the Royal Free Hospital and the Katharine Dormandy Trust for Haemophilia and Related Disorders.

The following people worked on the 2013 version of the project:

• Pavithra M. Rallapalli, UCL, London
• Geoffrey Kemball-Cook, Royal Free Hospital, London
• Edward G. Tuddenham, Royal Free Hospital, London
• Keith Gomez, Royal Free Hospital, London
• Stephen J. Perkins, UCL, London

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