Thrombocytopenia-absent radius syndrome.

Thrombocytopenia-absent radius (TAR) syndrome is a relatively uncommon condition characterized by absent radii with the presence of thumbs and congenital or early-onset thrombocytopenia that tends to resolve in childhood. The precise cause of this condition is unknown, although recently a microdeletion of chromosome 1q21.1 has been found in all investigated individuals. However, this microdeletion alone is not sufficient to cause TAR syndrome, and another, uncharacterized genetic alteration is thought to be involved in the pathogenesis.