Activation-induced cytidine deaminase: The missing piece of many puzzles.

Twenty-five years ago, two Cell papers reported the key missing functional piece in three molecular puzzles. The genetic swapping of immunoglobulin constant regions, the mutational fine-tuning of antibody specificity, and a baffling human immunodeficiency were traced to the action of one enzyme: activation-induced cytidine deaminase (AID).