Human patients with genetic defects in PRODH (HPI, L-Pro levels up to 10-fold higher than normal) or in P5CDH (ALDH4A1; HPII, L-Pro levels up to 15-fold higher and P5C excretion) suffer schizoaffective disorders and schizophrenia (Table 2; Liu et al., 2002; Bender et al., 2005; Raux et al., 2007; Clelland et al., 2011; Nagaoka et al., 2020). The gene discussed is ALDH4A1; the disease is schizophrenia.