Female monozygotic twins discordant for hemophilia A due to nonrandom X‐chromosome inactivation

We describe monozygotic female twins discordant for hemophilia A, born to a carrier mother and normal father. Affected twin A presented at age 1 year with excessive bruising and factor VIII procoagulant activity (FVIII:C) of less than 1% of normal. Twin B is an asymptomatic carrier with FVIII:C level of 42%. Peripheral blood DNA was tested for X‐chromosome inactivation (methylation) patterns of the X‐linked human androgen receptor gene, comparing the twins' patterns to parental. Twin A showed nonrandom inactivation skewed toward the paternal X, whereas twin B showed random X‐inactivation. This is the first reported case of discordance for hemophilia A between female monozygotic twins. Am. J. Hematol., 2008. © 2008 Wiley‐Liss, Inc.