In DNA testing, DNA is analyzed in what are known as Short Tandem Repeats or STRs. Using what is known as Polymerase Chain Reaction to amplify the DNA, the scientists will locate repeated sequences on the DNA molecule. The analysis takes short parts of the DNA and look at the sequence of base pairs. STR is usually done using nuclear DNA and thus, using the DNA found in the cell nucleus. The number of repeats on DNA may vary from person to person but the reason is not understood. However, differences in repeats on DNA profiles in people help to distinguish between people. The number of different repeats is miniscule from one person to the next as most of us share a large bulk of our DNA. However, it is those tiny differences which help establish or exclude biological relationships between people.