Registered and consented patients with moderate or severe HA or HB without inhibitors were included.

Here, we characterized an abnormal F8 due to atypical complex X chromosome rearrangements in a Japanese patient with severe HA.

Conclusion: We identified the aberrant X chromosome with a split F8 due to a multi-step rearrangement in a patient with severe HA.

Half of the patients with severe HA have a recurrent inversion in the X chromosome, that is, F8 intron 22 or intron 1 inversion.