Klenow fragment - a product of proteolytic digestion of the DNA polymerase I from E. coli; it has both polymerase and 3' -exo nuclease activities but not 5' exonuclease activity.

Km - dissociation constant that characterises the binding of an enzyme to a substrate. The smaller the value of K_m, the tighter the binding of the enzyme to the substrate. Also called the Michaelis constant.

Knockout - an animal resulting from an embryonic stem cell in which a normal functionalgene has been replaced by a non functional form of the gene. This technique isused extensively in mice much can be learned about the function of a gene bystudying the phenotype of animals that lack the peptide product of that gene.