Discordance for ankylosing spondylitis in a B27 negative monozygotic twin

A case in which only one sibling of an identical, HLA-B27 negative, twin pairing is affected by ankylosing spondylitis is reported. While endogenous variables may predispose certain populations to ankylosing spondylitis, the discordance illustrated in these seronegative monozygotic twins indicates that exogenous factors may certainly influence the development and course of this disease.