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Home > STR Data Analysis & Interpretation > Data Troubleshooting > Stutter
image of stutter on an electropherogram

Stutter is a by-product of the amplification of STR loci whereby a minor product one repeat smaller than the primary allele is generated. Sequence analysis of stutter products of STR loci has shown that the product is missing one core repeat unit relative to the main allele.02 Although the mechanism is not entirely understood, stutter occurs in a reproducible and predictable fashion. The proportion of the stutter product relative to the main allele (percent stutter) is measured by dividing the height (or area) of the stutter peak by the height (or area) of the main allele peak.01

Typically, stutter is effected by:

In known single source samples, stutter is identifiable by its size and position. However, with mixed samples, stutter and alleles can overlap, complicating interpretation.

The scientific community, as well as individual forensic laboratories, has conducted validation studies to determine the expected range of stutter percentages. In general, stutter percentages do not vary significantly.

There are two cases in which variability in the stutter percentages can be seen:

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NOTE TO USERS: The President’s DNA Initiative DNA Analyst Training program and assessment were completed and published in 2005, in cooperation with the National Institute of Justice. The science and techniques in the program are sound and proven, however, program content has not been updated to include tools and technologies developed and in use after 2005, including many kits and robots. Assessment questions address only content delivered in this program and may not contain the full range of tools in use in your laboratory.