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A short tandem repeat (STR) is a repetitive sequence of nucleotides in DNA where the repeating unit is relatively short, usually 2-6 base pairs long. These repeating units occur at specific locations in the genome and vary in the number of repeats from individual to individual. For example, the sequence might look like this for two different individuals: Person A: AGAGAGAG Person B: AGAGAGAGAGAG In this case, the repeating unit is "AG", and it repeats four times in Person A's DNA and six times in Person B's DNA.

The use of short tandem repeats (STRs) in forensics revolves around their high degree of polymorphism, meaning that these sequences differ among individuals. In human populations, STRs show significant variation in the number of repeat units, allowing for unique DNA profiling. In forensic science, a technique called polymerase chain reaction (PCR) is used to amplify specific STR regions from a DNA sample, such as those found at a crime scene or from a suspect. This amplified DNA is then separated and detected using gel electrophoresis or capillary electrophoresis, creating a DNA profile. The DNA profiles from the crime scene and the suspect can be compared to determine if they match or not. Multiple STR regions are typically analyzed in these profiles to increase the reliability and accuracy of the comparison. The more STRs match between the crime scene and suspect, the higher the likelihood that the DNA came from the same person. In summary, short tandem repeats are essential in forensic science for DNA profiling and comparing DNA samples. The variation found in these DNA sequences makes it possible to differentiate between individuals and provide vital information for criminal investigations.