Deletion of the factor IX gene as a result of translocation t(X;1) in a girl affected by haemophilia B

A balanced de novo translocation t(X;1) is described in a girl with severe haemophilia B. The translocated X was shown cytologically to be preferentia lly active, and methylation analysis of the DXS255 locus confirmed the skew ed X-inactivation with the paternal allele being the active one. Cytogeneti c and molecular analysis showed that this chromosomal rearrangement led to the deletion of at least part of the factor IX gene. Therefore, the girl wa s heterozygous for factor IX deficiency and expression of her clinical phen otype was the result of the inactivation of the normal maternal X chromosom e. The localization of one of the X chromosome translocation breakpoints in YAC clone 957F9, that was demonstrated to map distally to the factor IX ge ne, revealed the complexity of this chromosomal rearrangement.