In vitro proof of concept studies have now been published for T cell GT using lentiviral vector gene addition for XLP, CD40 ligand deficiency, IPEX, perforin deficiency and MUNC13-4 deficiency [the latter two are forms of familial haemophagocytic lymphohistiocytosis (HLH)] (Passerini et al., 2013; Hubbard et al., 2016; Soheili et al., 2016; Ghosh et al., 2018; Kohn et al., 2021). The gene discussed is PRF1; the disease is hemophagocytic syndrome.