For example, Zhang et al., who conducted a retrospective study on a cohort of 299 (173 children and 126 adults) COVID-19 patients, investigated the expression and distribution of ACE2 and lung progenitor cells and found that ACE2-positive cells and lung progenitor cells were decreased in the elderly, mainly in the lower pulmonary tract [28]. The gene discussed is ACE2; the disease is COVID-19.