ACE2 and COVID-19: The red cluster contained the most keywords (n = 46), including “stromal cells”, “mesenchymal stem cell”, “acute lung injury”, etc. The green cluster was followed (n = 35), which includes “COVID-19”, “therapy”, “transplantation”, etc. Next was the blue cluster (n = 33), including “SARS-CoV-2”, “coronavirus”, “cytokine storm”, “ace2”, etc. Finally, the yellow cluster (n = 25) contained “infection”, “expression”, “regeneration”, and so on.