Genetic analysis through whole exome sequencing (WES) was carried out in three patients, revealing two cases of activated phosphoinositide 3 kinase (PI3K)-δ syndrome (APDS) caused by heterozygous GOF mutations in PIK3CD, and one TACI deficiency due to heterozygous mutation in TNFRSF13B. Both patients with APDS exhibited decreased naïve T cells, reduced number of switched memory B cells, and very high percentage of transitional B cells (Table 2). The gene discussed is TNFRSF13B; the disease is activated PI3K-delta syndrome.