ADA and X-linked lymphoproliferative disease: The reason why some PID patients only manifest late in their lives may either lie in the variable expressivity of their respective mutation (e.g., as seen in ADA1), in the fact that a PID-predisposing condition needs to be triggered by a specific infection (e.g., EBV in XLP), or that the PID is not primarily monogenetic but has an epigenetic component (22).