STRs are important in current, forensic DNA testing. The abbreviation, STR stands for Short Tandem Repeat. STRs are the type of DNA used in most of the currently popular forensic DNA tests. STR is a generic term that describes any short, repeating DNA sequence. One of the more commonly encountered STR test designs in forensic testing is Multiplex STR. Such systems combine three or more different PCRs in one reaction that targets distinct STR loci at the same time. Three of the commonly used loci are called, CSF1PO, TPOX and THO1.

Multiplex STRs are often combined with PCR for another locus called, Amelogenin (adds little to the discriminating power of the test). Its purpose is to help distinguish male and female sources of DNA by detecting the X and Y chromosomes. The Amelogenin products have sizes that place them outside the size ranges of the other loci.

Multiplex STRs are technically more simple and direct at the allele detection stage. On the other hand, multiplex STR is slightly more vulnerable to missing alleles. There are two reasons for this.

• Larger DNA fragments are degraded before smaller ones. This is simply due to the fact that larger DNA molecules are bigger targets for degradative enzymes than smaller DNA molecules.
• PCR itself favors (will produce more of) smaller DNA targets compared to larger ones that take more time to copy. The copying is done by a protein called an enzyme. It can finish copying smaller DNA fragments more rapidly than larger ones.