The first phase of the paternity test consists in the extraction of DNA from biological samples taken for the examination. Within the DNA lies genetic information for protein synthesis.

The tract of the DNA containing a "piece" of genetic information is defined as a gene, and its location on the chromosomes is defined as a locus (plural: loci). The determination of the genetic profile of an individual involves the genotypization of 9-15 DNA regions (loci) whose length is highly polymorphic, variable from individual to individual, and known as Microsaletellite or STR (Short Tandem Repeat) regions.

Analysis of the microsatellites is conducted by means of an enzymatic reaction of DNA amplification known as Polymerase Chain Reaction (PCR); permits in in vitro amplification of a specific DNA region by copying it in various successive phases until millions of copies are obtained. In practice, if we consider the DNA molecule as a large book and the Microsatellites (STR) of the DNA as pages of this book: with the PCR method, we can "copy" this page millions of times until we obtain a quantity of DNA large enough to carry out the examination.

After the enzymatic amplification reaction, the genetic profile is automatically determined by use of an automatic DNA sequencer that permits a completely automated DNA profiling useful for paternity testing.