Applied Biosystems and Promega manufacture most forensic STR DNA amplification kits. More recently, Qiagen has also produced a commonly used STR DNA amplification kit.  Each manufacturer has modified their kits to meet the changing needs of the forensic science community. 

Read more about Locus Selection in this course.

Each manufacturer has independently developed different primer sets for the Combined DNA Index System (CODIS) core loci. It is possible, albeit rare, to have one set of primers successfully amplify a locus while another fails. The failures are primarily due to primer binding site mutations and the result is a null allele.⁰²

In general, a primer binding site mutation results in a homozygous profile for an actual heterozygote: this is called a "null allele". The manufacturers have introduced additional primer sets for their kits in an effort to minimize the occurrences of null alleles. When comparing DNA profiles developed from the different manufacturers' kits, it is important to take this into account.

For example, if a kit from manufacturer A types a locus as "12, 14" and a kit from manufacturer B types the same locus as "12" this difference may likely be due to a null allele.

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