ERCC2 and Cowden syndrome 1: The most severe form of CS, called COFS, is neonatal lethal.177 Mutations in XPB and XPD also contribute to XP/CS, a condition with overlapping symptoms of XP and CS.57 Symptoms include skin abnormalities associated with XP and severe neurological and developmental defects associated with CS.57 As shown in Table 3, the XPB missense mutations F99S and Q545STOP and the XPD missense mutation D681H are commonly associated with XP/CS; however, other frameshift mutations in these subunits have been documented.