For screening, we need to search for a cDNA clone encoding a protein. An antibody probe screens specific cDNA plasmid. It involves the use of an expression vector. Since we need to search for a cDNA clone encoding a protein, we use expression vectors. An expression vector is nothing but the vectors designed for the expression of the genes. When the cDNA gets cloned into an expression vector, the gene gets expressed and produces proteins. The cDNA gets inserted into an expression vector such that it gets fixed between the promoter and the termination signal. When cDNA gets inserted in the host, the gene gets expressed. The selection of the host involves E. coli cells. The expression vector containing cDNA gets inserted into the bacteria through the process of transformation. Why only expression vectors? An expression vector expresses the genes efficiently. Screening the cDNA clones requires a specific host such as E. coli. The growth of the cells helps us in identifying the recombinants and non-recombinants. It depends on the expression of the cDNA or the desired gene. Designing an expression vector depends on the type of organisms such as a prokaryote or a eukaryote. Hence, for every type of an organism, an expression vector design gets ready. Plus, we add a purification tag or a fusion partner along with the cDNA or a cloned gene. It helps in obtaining a purified product.