Transmembrane Protease Serine 2, or TMPRSS2, is an enzyme belonging to the family of serine proteases. Some coronaviruses, including SARS-CoV-2 (the virus responsible for the disease COVID-19), are activated by TMPRSS2, where TMPRSS2 functions to "prime" the S protein, enabling the spike protein to bind to ACE2 and therefore gain entry into the cell to begin viral replication. 

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