Adult lung is more vulnerable to infection due to the much higher expression of CTSL and CTSB and slightly higher expression of ACE2. Also, the different expression patterns of the COVID-19 factors (TMPRSS2, ACE2, MTHFD1, CTSL and CTSB) well demonstrates an explanation that the different clinical patterns between adult and children/infants. The gene discussed is CTSL; the disease is COVID-19.