USB1 and poikiloderma with neutropenia: Following discovery of the causative gene [6] and molecular evidence for distinct genetic control between PN and Rothmund-Thomson syndrome (RTS; OMIM#268400) [7], 31 PN patients have been tested and found to bear 17 different mutations in the responsible C16orf57 gene, 84% of the patients were found in the homozygous state [6,8-12] and only six were compound heterozygous [6,11,13].