The Molecular Basis of Severe Hemophilia B in a Girl

Abstract

Severe phenotypic expression of recessive X-linked disorders in females is rare, although examples have been reported. Cases have been documented in females who were homozygous for a recessive allele1 and in female patients with Turner's syndrome2 or a structural abnormality of the X chromosome.3 4 5 In females with apparently normal X chromosomes, "extreme lyonization" (that is, deviation from random X-chromosome inactivation) has been postulated to be the cause of severe clinical expression of X-linked recessive disorders.6 , 7 Nonrandom inactivation has been documented in females with structural X-chromosome abnormalities such as deletions8 , 9; inactivation of the abnormal X chromosome tends to minimize the . . .