PNP and immune system disorder: In another report, ATOs were used to evaluate the effects of purine nucleoside phosphorylase (PNP) inactivation, an enzyme mediating the breakdown and recycling of guanine nucleoside, whose deficiency in humans is associated with both immunodeficiency and autoimmunity (Al-Saud et al., 2020; Giblett et al., 1975; Markert, 1991).