Retroviral gene therapy in autologous HSCs has provided clinical benefit in several PIDs including SCID-X1 (Aiuti et al., 2002; Gaspar et al., 2004; Hacein-Bey-Abina et al., 2010; Gaspar et al., 2011), Adenosine deaminase deficiency (ADA-SCID) (Aiuti et al., 2009; Boztug et al., 2010; Cicalese et al., 2016; Scott et al., 2017), Wiskott Aldrich Syndrome (WAS) (Ott et al., 2006; Aiuti et al., 2013; Hacein-Bey Abina et al., 2015), and X-linked chronic granulomatous disease (X-CGD) (Cavazzana-Calvo et al., 2010; Kang et al., 2010). This evidence concerns the gene ADA and severe combined immunodeficiency.