**4.1. Autosomal STR profiling**

In forensic DNA typing, short tandem repeats (STR) or microsatellites are the most frequently genotyped in order to distinguish between individuals, to tie an individual to a crime or to exonerate the innocent. STRs were discovered in the 1980s [9] and since then, they are the "gold standard" in human identification in forensic investigations. They consist of mono‐, di‐, tri‐, tetra‐, penta‐, and hexa‐nucleotide repeat. An individual can be either homozygous (with the same number of repeats) or heterozygous (different number of repeats) in a certainly locus. Tetra‐nucleotide repeats are used for genotyping in forensic DNA analysis [10].

STR profiles obtained from biological samples found at crime scenes are compared with other profiles of known suspects and are identified by police or are included in a national forensic DNA database [11]. Also, the STR profiling is used in paternity/maternity testing, disaster victim identification (DVI), rape perpetrators identification, and kinship testing [12]. Due to the STR profiles, in numerous cases, the persons have been excluded from involvement in crimes and have been exonerate.

A main advantage of the STRs markers consists in the fact that they can test in a rapid, simple, and simultaneously way more than 10 STR loci by multiplexing [13]. Due to this characteristic, it offers an increased degree in the identification of different biological samples.

The tetra‐nucleotide and penta‐nucleotide systems are included in the multiplex analysis kits because they can provide results with an increased index of exclusion. The nomenclature of the STR loci and the allelic variants was established in 1993 by the DNA Commission of the International Society of Forensic Genetics (ISFG). Except their usefulness in forensic DNA analysis, STRs became used in medical genetic research, because it has been demonstrated that the trinucleotide STR loci is associated with some genetic disorders. The DNA profile refers to the genotype (the number of repeats found in each allele of the analyzed STR marker) of a suspect, victim, or crime scene sample.

In the development of STR typing system, in 1997, the Federal Bureau of Investigations (FBI) introduced the database named CODIS (Combined DNA Index System) that included 13 autosomal loci and the amelogenin sex test [14]. These loci are highly polymorphic, localized in non‐coding regions which are on different chromosomes. As an improvement in their efficiency, the new multiplexes that amplify 16 loci or more, in a single reaction (including amelogenin too), have been introduced in the last years [15]. The most common STR kits used in the forensic laboratories for the identification are manufactured by three companies: Life Technologies, Promega and Qiagen.

The forensic DNA analysis is made through multiplex PCR amplification of 10–16 STRs or more, followed by automated sequencing equipment, such as capillary electrophoresis (CE) [16].

The STR‐based forensic DNA analysis has been well accepted by population and professionals, as an important tool in human identifications and in the criminal justice.

Other sources of genetic variations that have been demonstrated to present more specialized uses in forensic identification are as follows: autosomal SNPs, the markers on the Y chromo‐ some and mitochondrial DNA (mtDNA).
