Apart from genetic testing, the number of diagnostic biomarkers is increasing. Frequently observed in patients with APDS, increased serum IgM levels might provide a starting point [43,44]. Flow cytometry is another useful tool that exposes distinctive immunologic anomalies, including lower numbers of naive T cells, poor memory B cells, and elevated transitional B cells [45]. Further clues into the functional effects of the mutations might be provided by other biomarkers, including hyperphosphorylation of AKT (a downstream target of PI3Kδ) [13]. This evidence concerns the gene AKT1 and activated PI3K-delta syndrome.