Cyclosporine is one of the most common and effective immunosuppressant drugs used to treat chronic diseases like arthritis and psoriasis, but it comes with a risk of serious side effects. Scientists think that may be because the drug broadly targets cyclophilins, a family of 17 regulatory proteins that play different roles in promoting cellular health. Although each individual cyclophilin subtype has a unique role, many current immunosuppressive drugs target the entire family, meaning that important unknown pathways may be accidentally turned off or otherwise altered.