Restriction endonucleases are bacterial enzymes that cleave double-stranded DNA (dsDNA) into smaller fragments. Each enzyme cleaves at a specific 4–8 base-pair sequence (a restriction site), producing DNA segments called restriction fragments. The sequences that are recognized are palindromic. Restriction enzymes form either staggered cuts (sticky ends) or blunt-end cuts on the DNA. Bacterial DNA ligases can join two DNA fragments from different sources if they have been cut by the same restriction endonuclease. This hybrid combination of two fragments is called a recombinant DNA molecule. Introduction of a foreign DNA molecule into a replicating cell permits the amplification (production of many copies) of the DNA, a process called cloning. A vector is a molecule of DNA to which the fragment of DNA to be cloned is joined. Vectors must be capable of autonomous replication within the host cell, must contain at least one specific nucleotide sequence recognized by a restriction endonuclease, and must carry at least one gene that confers the ability to select for the vector such as an antibiotic resistance gene. Prokaryotic organisms normally contain small, circular, extrachromosomal DNA molecules called plasmids that can serve as vectors. They can be readily isolated from the bacterium (or artificially constructed), joined with the DNA of interest, and reintroduced into the bacterium, which will replicate, thus making multiple copies of the hybrid plasmid. A DNA library is a collection of cloned restriction fragments of the DNA of an organism. A genomic library is a collection of fragments of dsDNA obtained by digestion of the total DNA of the organism with a restriction endonuclease and subsequent ligation to an appropriate vector. It ideally contains a copy of every DNA nucleotide sequence in the genome. In contrast, complementary DNA (cDNA) libraries contain only those DNA sequences that are complementary to processed messenger RNA (mRNA) molecules present in a cell and differ according to cell type and environmental conditions. Because cDNA has no introns, it can be cloned into an expression vector for the synthesis of human proteins by bacteria or eukaryotes. Cloned, then purified, fragments of DNA can be sequenced, for example, using the Sanger dideoxy chain termination method. A probe is a small piece of RNA or single-stranded DNA (usually labeled with a radioisotope, such as 32P, or another identifiable compound, such as biotin or a fluorescent dye) that has a nucleotide sequence complementary to the DNA molecule of interest (target DNA). Probes can be used to identify which clone of a library or which band on a gel contains the target DNA. Southern blotting is a technique that can be used to detect specific sequences present in DNA. The DNA is cleaved using a restriction endonuclease, after which the pieces are separated by gel electrophoresis and are denatured and transferred (blotted) to a nitrocellulose membrane for analysis. The fragment of interest is detected using a probe. The human genome contains many thousands of polymorphisms (DNA sequence variations at a given locus). Polymorphisms can arise from single-base changes and from tandem repeats. A polymorphism can serve as a genetic marker that can be followed through families. A restriction fragment length polymorphism (RFLP) is a genetic variant that can be observed by cleaving the DNA into restriction fragments using a restriction enzyme. A base substitution in one or more nucleotides at a restriction site can render the site unrecognizable by a particular restriction endonuclease. A new restriction site also can be created by the same mechanism. In either case, cleavage with the endonuclease results in fragments of lengths differing from the normal that can be detected by hybridization with a probe. RFLP analysis can be used to diagnose genetic diseases early in the gestation of a fetus. The polymerase chain reaction (PCR), another method for amplifying a selected DNA sequence, does not rely on the biologic cloning method. PCR permits the synthesis of millions of copies of a specific nucleotide sequence in a few hours. It can amplify the sequence, even when the targeted sequence makes up less than one part in a million of the total initial sample. The method can be used to amplify DNA sequences from any source. Applications of the PCR technique include 1) efficient comparison of a normal gene with a mutant form of the gene, 2) forensic analysis of DNA samples, 3) detection of low-abundance nucleic acid sequences, and 4) prenatal diagnosis and carrier detection (for example, of cystic fibrosis). The products of gene expression (mRNA and proteins) can be measured by techniques such as northern blots, which are like Southern blots except that the sample contains a mixture of mRNA molecules that are separated by electrophoresis, then hybridized to a radiolabeled probe; microarrays are used to determine the differing patterns of gene expression in two different types of cells (for example, normal and cancer cells); enzyme-linked immunosorbent assays (ELISA); and western blots (immunoblots) are used to detect specific proteins. Proteomics is the study of all the proteins expressed by a genome. The goal of gene therapy is the insertion of a normal cloned gene to replace a defective gene in a somatic cell, whereas the goal of gene editing is the repair of a mutated gene. Insertion of a foreign gene (transgene) into the germline of an animal creates a transgenic animal that can produce therapeutic proteins or serve as gene knockin or knockout models for human diseases.