Isolation of an active human transposable element.

Two de novo insertions of truncated L1 elements into the factor VIII gene on the X chromosome have been identified that produced hemophilia A. A full-length L1 element that is the likely progenitor of one of these insertions was isolated by its sequence identity to the factor VIII insertion. This L1 element contains two open-reading frames and is one of at least four alleles of a locus on chromosome 22 that has been occupied by an L1 element for at least 6 million years.