Table 1.

Studies reporting ACE2 or TMPRSS2 head and neck expression

Authors	Design	Samples	Methods	ACE2 & TMPRSS2 expression	Findings
Vaarala	Mixed	Human	TMPRSS2	Salivary glands	1. TMPRSS2 is expressed in human salivary gland tissues
2001 [10]	Study	Mouse	RNAseq	TMPRSS2 (human):+
Hamming	Human	Human	ACE2	Oral, Nasal, Nasopharyngeal Epithelium & endothelium	1. ACE2 was found in endothelial (arteries & veins) and epithelial cells of nasal, rhinopharyngeal and oral mucosa. Precisely, the epithelium expression concerned the basal layer cells
2004 [11]	Study		IHC	ACE2 human:+ (all mucosa)
Jia	Human	Human	ACE2	Tracheal Epithelium & endothelium	1. ACE2 was more expressed on the apical than the basolateral surface of polarized airway epithelia
2005 [12]	Study		IHC & biotinylation	ACE2 (human):+
Liu	Experimenal	Monkey	ACE2	Naso- Oro- Hypopharyngeal & Tracheal Epithelium	1. ACE2 is expressed in salivary gland ducts of the pharyngeal glands
2011 [13]	Study		IHC	ACE2 Monkey:+	2. ACE2 was expressed in epithelial cells (lamina propria, respiratory tract)
				Virion in Saliva:+	3. Virus was found in saliva of infected monkeys
Bilinska	Animal	Mouse	ACE2 & TMPRSS2	Olfactory Epithelium	1. ACE2 & TMPRSS2 are expressed in sustentacular cells of the olfactory epithelium but not/much less in most olfactory receptor neurons
2020 [14]	Study		RNAseq, RT-PCR,	ACE2: sustentacular cells
			In situ hybridization,	TMPRSS2: sustentacular cells	2. Expression of the entry proteins increases in animals of old age
			WB & IHC
Brann	Mixed	Mouse	ACE2 & TMPRSS2	Olfactory Epithelium	1. In human, ACE2 & TMPRSS2 were not identified in purified olfactory neurons
2020 [15]	Study	Human	RNAseq	ACE2 & TMPRSS2 Mouse: non-neuronal cells
				ACE2 & TMPRSS2 Human: glial and neuronal stem cells	2. ACE2 was identified in glial cells & olfactory stem cells
Butowt	Mixed	Mouse	ACE2 & TMPRSS2	Olfactory Epithelium	1. Nasal epithelial cells have lower levels of ACE2 & TMPRSS2 compared with epithelial cells of the lower respiratory tract
2020 [16]	Study	Human	RNAseq	ACE2 Mouse & Human: non neuronal cells
				TMPRSS2 Mouse & Human: neuronal & non-neuronal cells	2. ACE2 has non-neuronal expression in olfactory epithelium
				Respiratory Epithelium	3. The expression of ACE2 & TMPRSS2 (mouse) were increased in elderly mouse
				ACE2 & TMPRSS2 Human:+ (Lower Airway > Nasal)
Cao	Human	Human	ACE2 gene	No localization provided	1.15 unique expression quantitative trait loci variants were found for ACE2
2020 [17]	Study		Genetic Analyzis		2. The genotypes of ACE2 gene polymorphism may be characterized by higher expression levels of ACE2 in East Asian population
					3. There would be different susceptibility or response to SARS-CoV2 in different populations
Chen	Human	Human	ACE2	Salivary glands	1. ACE2 is expressed in human granular cells of salivary glands
2020 [18]	Study		RNAseq	ACE2 human:+
Hikmet	Human	Human	ACE2	Nasopharyngeal Epithelium	1. There was no ACE2 expression in nasopharyngeal cells
2020 [19]	Study		IHC	ACE2 (human): no expression in nasopharynx
Lee	Human	Human	ACE2	Tracheal, Nasal & Sinusal Epithelium	1. ACE2 is expressed in ciliated epithelial cells (cilia organelle)
2020 [20]	Study		IHC	ACE2 (human):+	2.There was no ACE2 expression in the non-ciliated goblet cells
					3. ACE2 expression is influenced by patient demographics, clinical characteristics, co-morbidities, or medication use. The use of ACE inhibitor drugs did not increase ACE2 protein expression
Li	Human	Human	ACE2	Thyroid	1. ACE2 is expressed by thyroid cells
Hikmet	Human	Human	ACE2	Nasopharyngeal Epithelium	1. There was no ACE2 expression in nasopharyngeal cells
2020 [19]	Study		IHC	ACE2 (human): no expression in nasopharynx
2020 [21]	Study		RNAseq	ACE2 human:+
Sungnak	Human	Human	ACE2 & TMPRSS2	Airway & Nasal epithelium	1. ACE2 was expressed in airway epithelial cells
2020 [22]	Study		RNAseq	ACE2 human:+	2. ACE2 is more expressed in nasal epithelial cells compared with other respiratory cells (goblet & ciliated cells)
				TMPRSS2 (human): subset of ACE2 cells
					3. TMPRSS2 is only expressed in a subset of ACE2 + cells
Xu	Human	Human	ACE2 & TMPRSS2	Oral Epithelium	1. ACE2 is expressed in the oral cavity epithelial cells
2020 [23]	Study		RNAseq	ACE2 human:+	2. ACE2 expression was higher in tongue than buccal and gingival tissues
				Oral T cells, B cells & fibroblasts
				ACE2 human:+
Xu	Human	Human	ACE2	Minor salivary glands	1. ACE2 is expressed in minor salivary glands
2020 [24]	Study		RNAseq	ACE2 human:+
Wu	Human	Human	ACE2	Nasal & Oral Epithelium	1. ACE is expressed in nasal epithelial cells
2020 [25]	Study		RNAseq	ACE2 (human):+	2. The was a higher virus concentration in the nasal-swab comparing with throat-swab, which is attributed to ACE2-expression in nasal epithelial cells
Ziegler	Mixed	Human	ACE2 & TMPRSS2	Sinusal & Nasal goblet epithelial cells	1. ACE2 & TMPRSS2 are co-expressed in nasal goblet secretory cells
2020 [26]	Study	Monkey	RNAseq	ACE2 & TMPRSS2 Human:+

ACE2 Angiotensin Converting Enzyme-2, IHC Immunohistochemistry, RT-PCR reverse transcription polymerase chain reaction, SARS-CoV-2 severe acute respiratory syndrome coronavirus-2, TMPRSS2 transmembrane protease serine-2, WB Western Blotting