Modern-day molecular biology relies on modular manipulations of an organism’s genome to study the behaviors of individual genes. Owing to the stereotyped organization of DNA across species, it is possible to cut-and-paste portions of one organism’s genome into the machinery and language of another organism to exploit improved expression. Generally, cloning refers to the replication of DNA by an organism other than the organism-of-origin. Cloning has become the cornerstone technique of molecular biology and has broad applications. These include the isolation of single genes, the preparation of modified versions of genes and overexpression studies, whether for phenotype analysis or protein production. In contrast to PCR, where a segment of DNA is copied in a cell-free reaction, molecular cloning refers to the use of a living organism, typically bacteria or yeast, to carry out the replication reaction.