AbstractThis paper describes the current state of knowledge of methods for analysing gene structure and localization. Illustrations are given of the preparation of complementary DNA libraries and their screening by positive‐negative selection, the use of synthetic oligodeoxynucleotides and the use of antibodies. Analysis of the EGF precursor is used as an example to show its close relationship to plasma membrane receptor and its homology with the LDL receptor. Analysis of cloned genome DNA by use of bacteriophage lambda or cosmids gives useful information about gene regulation and evolution. Mutations by frame shift, point or missence mutations are discussed with reference to the LDL receptor and the apolipoproteins. The techniques of gene mapping by rat‐human cell hybridization and hybridizationin situ are illustrated, again with reference to genes coding for enzymes of cholesterol metabolism, the apolipoproteins and insulin‐like growth factors. Finally the potential ofin vitro mutagenesis and the injection of cloned DNA into the fertilized mouse ovum are discussed.