Grandpaternal mosaicism in a family with isolated haemophilia A

About one third of cases of haemophilia A have no family history of the dis order, and 20% are thought to be due to a new mutation. In the family repor ted here, a 3 bp deletion was detected in DNA from the proband at the 3' en d of exon 15. Direct sequencing of genomic DNA prepared from blood and bucc al cells of the grandfather revealed both normal and mutant sequences, sugg esting that he is a mosaic for this mutation. This highlights the usefulnes s of mutation detection, both for accurate genetic counselling and to deter mine the origin of new mutations of haemophilia.