PIK3CD mutation-related immunodeficiency disease, which is also known as activated phosphoinositide 3-kinase δ syndrome (APDS)-1, and which is mainly due to an overactivated PI3K/Akt/mTOR signaling pathway, can be treated from the cause using sirolimus, an mTOR inhibitor (Luo et al., 2018; Kang et al., 2020; Singh et al., 2020; Redenbaugh and Coulter, 2021; Rivalta et al., 2021). The gene discussed is PIK3CD; the disease is Down syndrome.