Chua et al. [26] conducted single-cell RNA sequencing of respiratory tract samples from severe coronavirus disease 2019 (COVID-19) patients and found that the expression of chemokines and their receptors in different cell populations significantly increased, including CCL2, CCL3, CCL20, CXCL1, CXCL3, and CXCL10. The gene discussed is CCL2; the disease is COVID-19.