In order for viruses to infect a host cell, they first must recognize a protein or receptor on the host cell surface. This process is called viral recognition. In the case of SARS-CoV-2, which causes the disease COVID-19, the virus recognizes the enzyme ACE2. Specifically, the S1 spike protein of SARS-CoV-2 binds the enzymatic domain of ACE2. Once S1 and ACE2 interact, the virus enters the host cell, mediated by the TMPRSS2 protease. BioLegend provides specific SARS-CoV-2 and host recombinant proteins and antibodies to help investigate this step of the disease.