References for this manuscript were identified through searches of PubMed using the following terms: ‘vitamin D’, ‘cholecalciferol’, ‘calcifediol’, ‘calcidiol’, ‘dipeptidyl peptidase-4’, ‘DPP-4’ and ‘DPP-4 inhibitors’ in combination with ‘COVID-19’, ‘ACE2’, ‘SARS-CoV-2’, ‘cytokine storm’ and ‘cytokine-release syndrome’. The gene discussed is ACE2; the disease is COVID-19.