Differences in the sequence of a single gene may be partly responsible for causing around 2% of relatively common autoimmune disorders including diabetes and arthritis. The gene codes for an enzyme called sialic acid acetylesterase (SIAE) that regulates the immune system's B cells — the cells responsible for producing antibodies against foreign invaders. In 24 of 923 people with conditions such as Crohn's disease, type 1 diabetes, rheumatoid arthritis, systemic lupus erythematosus and multiple sclerosis, the gene was present in a variant form. For the past five years, genome-wide screens of large groups of patients have searched for commonly occurring...