Medical Genetics The defect can lead to a missing enzyme(eg, adenosine deaminase deficiency), a missing protein(eg complement component deficiencies) or developmental arrest at a specific differential stage(eg, pre-B-cell arrest in X-linked agammaglobulinemia)Medical Genetics The defect can lead to a missing enzyme (eg, adenosine deaminase deficiency), a missing protein (eg, complement component deficiencies), or developmental arrest at a specific differential stage (eg, pre-B-cell arrest in X-linked agammaglobulinemia)