(angiotensin converting enzyme 2) OR (ACE2) OR (ACEH) OR (Renin Angiotensin System) OR (Angiotensin I converting Enzyme 2) AND (COVID-19) OR (Coronavirus) AND (head and neck) AND (oral cavity) OR (mouth) AND (saliva) OR (salivary gland) AND (oropharynx) AND (nasopharynx) OR (nasal cavity) AND (tongue) AND (buccal mucosa) AND (cancer) OR (carcinoma) AND (oral squamous cell carcinoma) AND (oral potentially malignant disorders) AND (thyroid) AND (single cell RNA sequencing) AND (immunohistochemistry) were used. Here, ACE2 is linked to carcinoma.