The SARS-CoV-2 virus, the cause of the current Covid-19 pandemic, is a member of the coronavirus family. Coronaviruses are enveloped viruses with an RNA-positive genome and a helically symmetrical nucleocapsid. There are 3 or 4 viral proteins in the membrane of coronaviruses. The most abundant structural protein is the membrane glycoprotein or M protein. The M protein crosses the membrane bilayer three times, leaving a short NH2-terminal domain outside the virus and a long COOH-terminal domain (cytoplasmic domain) inside the virion. This long carboxy-terminal domain can interact with the nucleocapsid protein (N protein). The M protein plays an important role in the intracellular formation of viral particles. Since the M protein is the most abundant protein on the surface of the virus, antibodies against this protein are found in the serum of patients. The M protein is therefore a good candidate as a diagnostic tool.