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STR Sequence Analysis for Characterizing Normal, Variant, and Null Alleles



Margaret C. Kline, Carolyn R. Steffen, John M. Butler, Amy Decker


DNA sequence variation is known to exist in and around the repeat region of short tandem repeat (STR) loci used in human identity testing. While the vast majority of STR alleles measured in forensic DNA laboratories worldwide type as "normal" alleles compared with STR kit allelic ladders, a number of variant alleles have been reported. In addition, a sequence difference at a polymerase chain reaction (PCR) primer binding site in the DNA template can cause allele drop-out (i.e., a "null" or "silent" allele) with one set of primers and not with another. Our group at the National Institute of Standards and Technology (NIST) has been sequencing variant and null alleles supplied by forensic labs and cataloging this information on the NIST STRBase website for the past decade. The PCR primer sequences and strategy used for our STR allele sequencing work involving 23 autosomal STRs and 17 Y-chromosome STRs are described along with the results from 113 variant and 16 null alleles.
Forensic Science International: Genetics


short tandem repeat, STR typing, DNA sequencing, allele dropout, null allele, variant allele


Kline, M. , Steffen, C. , Butler, J. and Decker, A. (2011), STR Sequence Analysis for Characterizing Normal, Variant, and Null Alleles, Forensic Science International: Genetics, [online], (Accessed June 16, 2024)


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Created July 31, 2011, Updated February 19, 2017