An expression vector is also called an expression construct and is usually a plasmid derivative designed for protein expression in cells.Protein expression refers to the technique in which proteins are regulated, modified and synthesized in living organisms or cells.For use in biotechnology products and proteinproduction,expression vectors are the basic tools. For example, insulin is used in diabetes. An expression vector has features such as a selectable marker, origin of replication, and a suitable site for the insertion of a gene (multiple cloning site). An expression vector must have necessary elements such as strong promoter, initiation sequence and a transcription termination sequence.Advancement in protein expression and introduction of advanced technologies in the field of genetics, molecular biology, genetic engineering and computer sciences drives the growth of expression vector market. Some medical applications of protein expression include the large scale production of human insulin and various other protein based products. Major protein expression systems include, mammalian cell expression systems, baculovirus based systems, K. lactis systems, E. coli systems, saccharomyces systems, Pichia systems and others.