Due to problems, such as the lack of widespread clinical lymphocyte subset testing, and the lagging development of flow cytometry in some medical institutions, most studies of T-lymphocytes in patients with COVID-19 have been limited to the detection of CD3+ T-lymphocytes, CD4+ T-lymphocytes, CD8+ T-lymphocytes, and the CD4+/CD8+ ratio [42]. The gene discussed is CD4; the disease is COVID-19.