an A-to-G intronic mutation located three base pairs upstream of exon 4 (intron-exon boundary IVS3-3A>G) in the F9 gene.One of Alexei's sisters, probably Anastasia, was a carrier, as, obviously, was their mother.
This is taken to be "a severe form of hemophilia B" by the authors, reporting in the 6 November Science.
It wasn't only Alexei's hemophilia that brought down the Tsars, but the attention and concern it drew from his parents distracted them from their duties as heads of state. Just one amino-acid substitution. Gives new meaning to that old chestnut about "for want of a nail, the kingdom was lost."