Hemophilia B in a 46,XX female probably caused by non-random X inactivation

A female whose father had severe hemophilia B was found to have a factor IX activity of about 1%. No chromosomal abnormality could be detected and DNA analysis gave no indications of deletions or mutations of Taq I cleavage sites in the factor IX gene. Analysis of the methylation pattern of locus DXS255 indicates that the expression of hemophilia B in this patient is caused by non-random X inactivation.