The term "restriction endonuclease mapping" covers a number of related techniques used to identify specific restriction enzyme recognition sites on small DNA molecules. A method for restriction endonuclease mapping of a 1,000-basepair (bp)-long DNA molecule is described in the fictitious experiment of this test. The most important fact needed to be considered to solve this test is the reaction catalyzed by polynucleotide kinase. This activity implies that polynucleotide kinase does not phosphorylate circular DNA molecules. (Contains 3 figures.)