again at very high speed for several hours in dense solutions of sugars or salts that form a density gradient, from low to high, in the tube during the centrifugation. In some cases, preformed gradients are used where solutions of steadily decreasing density are carefully overlaid on each other. Like an object in the Dead Sea, despite the centrifugal force the virus particles cannot sink into solutions that are more dense than they are and they form discrete layers of, often visible, concentrated viruses in the tube. Caesium chloride is often used for these solutions as it is relatively inert but easily self-forms a gradient when centrifuged at high speed in an ultracentrifuge. Buoyant density centrifugation can also be used to purify the components of viruses such as their nucleic acids or proteins. Electrophoresis The separation of molecules based on their electric charge is called electrophoresis. Viruses and all their components can be separated and purified using this method. This is usually done in a supporting medium such as agarose and polyacrylamide gels. The separated molecules are revealed using stains such as coomasie blue, for proteins, or ethidium bromide for nucleic acids. In some instances the viral components are rendered radioactive before electrophoresis an are revealed using photographic film in a process known as autoradiography. Sequencing of viral genomes As most viruses are too small to be seen by a light microscope, sequencing is one of the main tools in virology to identify and study the virus. Traditional Sanger sequencing and next-generation sequencing (NGS) are used to sequence viruses in basic and clinical research, as well as for the diagnosis of emerging viral infections, molecular epidemiology of viral pathogens, and drug-resistance testing. There are more than 2.3 million unique viral sequences in GenBank. NGS has surpassed traditional Sanger as the most popular approach for generating viral genomes. Viral genome sequencing as become a central method in viral epidemiology and viral classification. Phylogenetic analysis Data from the sequencing of viral genomes can be used to determine evolutionary relationships and this is called phylogenetic analysis. Software, such as PHYLIP, is used to draw phylogenetic trees. This analysis is also used in studying the spread of viral infections in communities (epidemiology). Cloning When purified viruses or viral components are needed for diagnostic tests or vaccines, cloning can be used instead of growing the viruses. At the start of the COVID-19 pandemic the availability