Presence of an IgM⁺IgD⁺CD27⁺ subset in hyper-IgM patients. Patients AID.1 and 40.1 have been reported previously.^20,21  Control is an 11-year-old child, age-matched with patient 40.1. Since IgD⁺CD27⁺cells coexpress IgM, the IgM⁺IgD⁺CD27⁺ subset is analyzed after IgD, CD27, and CD19 labeling of purified B cells. IgD and CD27 expression is shown after gating on CD19⁺ cells. Percentages of cells in the naive and the 2 CD27⁺ quadrants are indicated.