DNA fingerprinting relies on the fact that DNA can be extremely variable in sequence from one individual to another and from one organism to another. Thus, the method has been used to identify individuals in forensic and paternity cases and to study genetic lineages of closely related organisms. These applications are discussed in the text and the latter application is illustrated in this experiment. In the exercise, students are given two DNA samples which are identified in the instructor guide. One sample is from sheep and the other from cow. In part A, the DNAs are digested with restriction enzymes and the sizes of the bands composed of highly repeated sequences are determined by comparison to standard DNAs of known lengths as shown below. In part B, the nonrepeated sequences are eliminated by a simple novel procedure. This step serves to enhance the banding patterns of the repeated sequences as shown in the gel on the right. From the results of this analysis, students determine which DNA sample is from cow and which is from sheep. This exercise requiring two 2-3 hour laboratory periods is the only authentic (non-simulated) DNA fingerprinting exercise available for the teaching laboratory.