The term "DNA fingerprint" has been used to describe the extensive restriction fragment length polymorphism associated with hypervariable minisatellites present in the human genome. Until now, it was necessary to hybridize Southern blots to specific probes cloned from human genomic DNA in order to obtain individual-specific restriction patterns. The present study describes the surprising finding that the insertfree, wild-type M13 bacteriophage detects hypervariable minisatellites in human and in animal DNA, provided no competitor DNA is used during hybridization. The effective sequence in M13 was traced to two dusters of 15-base pair repeats within the protein III gene of the bacteriophage. This unexpected use of M13 renders the DNA fingerprinting technology more readily available to molecular biology laboratories.E XTENSIVE RESTRICTION FRAGMENT length polymorphism (RFLP) associated with moderately repeated sequences has recently been described in the human genome (1-4). The hypervariable nature of these "minisatellites" has been exploited as a means of developing a powerfiul fingerprinting technique with applications in paternity testing, forensic medicine, and mapping of the human genome (5, 6). The probes used in such determinations were originally derived from segments of DNA discovered by chance within anonymous (3) or gene-associated pieces of genomic DNA (1, 4, 7). We report here that a sequence present in the protein III gene of the widely used M13 bacteriophage vector (8) allows detection of a distinct set of hypervariable minisatellites in human and animal DNA.In the course of a systematic search of RFLPs associated with the thyroglobulin gene, we observed that a set of probes corresponding to DNA segments subcloned in the 'bacteriophage M13 gave different results when used in the classical hybridization mixture [Denhardt's medium plus herring sperm DNA (9) tematically observed with the enzyme Hae III in the latter condition. It soon became evident that the pattern was unrelated to the nature of the insert and that wild-type M13 bacteriophages gave the same results. Nine unrelated individuals displayed different pattems while monozygotic twins were indistinguishable (Fig. 1, A and B). Similar blots hybridized with an M13 probe in the presence of herring sperm DNA showed no polymorphism (Fig. 1C). The logical explanation for this finding was that a segment in M13 hybridized to a hypervariable minisatellite that could be competed for by fish DNA. Ifthis were true,