Expression vector refers to a plasmid or other vehicle that has been specifically engineered to facilitate the expression of a gene or a particular protein. It involves a complex and specialized skillset that includes the design, construction, and optimization of vectors, as well as the selection of appropriate host cells and gene expression systems. This technique is widely used in molecular biology research and biotechnology applications, allowing scientists to study and manipulate the expression of genes and proteins for various purposes, including disease diagnosis and therapy, and the production of recombinant proteins for medical or industrial uses.