ACE2 and COVID-19: Cross-species and human-to-human transmission of COVID-19 is mainly dependent on spike protein receptor-binding domain and its host receptor ACE2.23  43 High expression of ACE2 was identified in lung (type II alveolar cells), oesophagus, ileum, colon, kidney (proximal convoluted tubules), myocardium, bladder (urothelial cells) and also recently the oral mucosa.