Patients with COVID-19 showed lower levels of tryptophan, correlated with the severity of the infection, affecting the production of kynurenine (metabolite of the amino acid tryptophan) and the activity of the enzyme IDO1 (present especially in immune cells such as macrophages that catalyze the degradation of tryptophan into kynurenine), impacting inflammatory responses during illness [36]. This evidence concerns the gene IDO1 and COVID-19.