In the previous study (Kido et al., 2021d), we reported the variants of CPS1, ASS, ASL, and OTC detected in the patients with urea cycle disorders through a nation-wide survey in Japan and suggested that the onset time and severity in Japanese patients with OTCD can be estimated based on the type of OTC gene variant that they carry, thereby demonstrating a genotype–phenotype correlation in OTCD. The gene discussed is ASS1; the disease is ornithine carbamoyltransferase deficiency.